Improve verify-tests-fail-without-fix Skill (#33513)

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## Description

This PR improves the `verify-tests-fail-without-fix` skill by fixing
fork repository detection, improving error handling, and making full
verification the recommended/default behavior.

## Changes

### 1. **Added `git fetch origin` for Accurate Diffs**
- Fetches the base branch from origin before computing git diff
- Ensures accurate fix file detection even in fork repositories
- Prevents false negatives when base branch is stale locally

### 2. **Improved Verification Mode Behavior**
- **Made full verification the primary/default mode** - tests must FAIL
without fix and PASS with fix
- **Added `-RequireFullVerification` parameter** - prevents silent
fallback to verify-only mode
- **Improved error handling** - script now warns/errors if no fix files
detected (instead of silently switching modes)
- **Verify-only mode still available** - runs when no fix files detected
AND `-RequireFullVerification` not set
- **Clearer expectations** - user explicitly controls whether
verify-only fallback is allowed

### 3. **Improved Documentation**
- Updated SKILL.md to reflect improved mode behavior
- Clarified requirements (fix files + test files for full verification)
- Updated mode selection table to show both modes clearly
- Added troubleshooting guidance for "no fix files detected" scenario

### 4. **Better Error Handling**
- Script exits early with actionable error if no fix files found AND
`-RequireFullVerification` set
- Provides explicit solutions (use `-FixFiles` or `-BaseBranch`
explicitly)
- Reduced ambiguity - user knows exactly what mode is running

## Why These Changes?

### Problem 1: Fork Repository Detection
When working in a fork, the base branch might not be up-to-date locally,
causing `git diff` to produce inaccurate results. This led to:
- Missing fix files in the diff
- False "only test files" mode activation
- Incorrect verification results

**Solution**: Always fetch the base branch from origin before computing
diff.

### Problem 2: Confusing Auto-Detection
The previous auto-detection system was unclear:
- Silent fallback to "verify failure only" when no fix files detected
- Users didn't know which mode was running
- Made debugging harder

**Solution**: 
- Add `-RequireFullVerification` parameter to make expectations explicit
- Error immediately if user expects full verification but no fix files
found
- Maintain verify-only mode for legitimate test-first workflows

### Problem 3: Silent Failures
When no fix files were detected (due to stale base branch or wrong base
detection), the script would silently fall back to "verify failure only"
mode, which wasn't the intended behavior.

**Solution**: Fail fast with clear error message and troubleshooting
steps when `-RequireFullVerification` is set.

## Testing

Verified the changes work correctly with:
- ✅ Fork repositories (fetch ensures accurate base branch)
- ✅ PRs with fix files + test files (full verification runs)
- ✅ Clear error when no fix files detected with
`-RequireFullVerification`
- ✅ Verify-only mode still works when appropriate (without the flag)
- ✅ `-RequireFullVerification` parameter enforces strict mode

## Example Usage

```bash
# Recommended for PR validation - ensures full verification
pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform android -RequireFullVerification

# With explicit test filter
pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform ios -TestFilter "Issue33356" -RequireFullVerification

# Override fix files or base branch if auto-detection fails
pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform android -FixFiles @("src/Core/File.cs") -BaseBranch "main"

# Test-first workflow (verify tests fail before writing fix)
pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform ios
```

## Breaking Changes

⚠️ **Behavior Change**: The skill now requires explicit intent when fix
files are not detected:
- **Without `-RequireFullVerification`**: Falls back to verify-only mode
(tests only need to FAIL)
- **With `-RequireFullVerification`**: Errors immediately if no fix
files found

**Recommended usage**: Always use `-RequireFullVerification` when
validating PRs to ensure full verification (tests FAIL without fix, PASS
with fix).

**Migration**: Update any automated scripts that call this skill to add
`-RequireFullVerification` for PR validation workflows.

## Related

- Part of improving PR verification workflows
- Complements the `pr` custom agent for better PR validation
- Ensures consistent behavior across fork and non-fork repositories
```

---

## Key Changes from Current Description

### Change 1: Section 2 Title and Content

**Current (inaccurate):**
```markdown
### 2. **Simplified Verification Mode**
- **Removed** "verify failure only" mode (when only test files exist)
```

**Corrected:**
```markdown
### 2. **Improved Verification Mode Behavior**
- **Made full verification the primary/default mode** - tests must FAIL
without fix and PASS with fix
- **Verify-only mode still available** - runs when no fix files detected
AND `-RequireFullVerification` not set
```

### Change 2: Breaking Changes Section

**Current (inaccurate):**
```markdown
⚠️ **Behavior Change**: The skill no longer supports "verify failure
only" mode.
```

**Corrected:**
```markdown
⚠️ **Behavior Change**: The skill now requires explicit intent when fix
files are not detected:
- **Without `-RequireFullVerification`**: Falls back to verify-only mode
(tests only need to FAIL)
- **With `-RequireFullVerification`**: Errors immediately if no fix
files found
```

### Change 3: Added Test-First Workflow Example

**Added:**
```bash
# Test-first workflow (verify tests fail before writing fix)
pwsh
.github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1
-Platform ios
```

This clarifies that verify-only mode is still useful for legitimate workflows.

---

## Why This Correction Matters

**Code Reality**: The implementation shows verify-only mode still exists at lines 173-193:
```powershell
if ($DetectedFixFiles.Count -eq 0) {
Write-Host "║ VERIFY FAILURE ONLY MODE ║"
    # ... mode implementation
}
```

**Impact of Inaccuracy**: 
- Future maintainers reading "removed" will be confused when they see the mode in code
- Users might think they can't use verify-only mode for test-first workflows
- Documentation doesn't match reality

**Correction Approach**:
- Acknowledge mode still exists
- Explain it's no longer the silent fallback (key improvement)
- Show it's still useful for test-first development
- Emphasize full verification is now recommended/default

---

---------

Co-authored-by: Shane Neuville <shneuvil@microsoft.com>

Author: kubaflo

.github/agents/pr.md

@@ -407,7 +407,7 @@ Tests were already verified to FAIL in Phase 2. Gate is a confirmation step:
 Use full verification mode - tests should FAIL without fix, PASS with fix.
 
 ```bash
-pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform android
+pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform android -RequireFullVerification
 ```
 
 ### Expected Output (PR with fix)

.github/skills/verify-tests-fail-without-fix/SKILL.md

@@ -1,89 +1,116 @@
 ---
 name: verify-tests-fail-without-fix
-description: Verifies UI tests catch the bug. Auto-detects mode based on git diff - if fix files exist, verifies FAIL without fix and PASS with fix. If only test files, verifies tests FAIL.
+description: Verifies UI tests catch the bug. Supports two modes - verify failure only (test creation) or full verification (test + fix validation).
 ---
 
 # Verify Tests Fail Without Fix
 
-Verifies UI tests actually catch the issue. **Mode is auto-detected based on git diff.**
+Verifies UI tests actually catch the issue. Supports two workflow modes:
 
-## Usage
+## Mode 1: Verify Failure Only (Test Creation)
+
+Use when **creating tests before writing a fix**:
+- Runs tests to verify they **FAIL** (proving they catch the bug)
+- No fix files required
+- Perfect for test-first development
 
 ```bash
-# Auto-detects everything - just specify platform
+# Auto-detect test filter from changed test files
 pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform android
 
 # With explicit test filter
 pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform ios -TestFilter "Issue33356"
 ```
 
-## Auto-Detection
+## Mode 2: Full Verification (Fix Validation)
+
+Use when **validating both tests and fix**:
+1. **Without fix** - tests should FAIL (bug is present)
+2. **With fix** - tests should PASS (bug is fixed)
+
+```bash
+# Auto-detect everything (recommended)
+pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform android -RequireFullVerification
 
-The script automatically determines the mode:
+# With explicit test filter
+pwsh .github/skills/verify-tests-fail-without-fix/scripts/verify-tests-fail.ps1 -Platform ios -TestFilter "Issue33356" -RequireFullVerification
+```
 
-| Changed Files | Mode | Behavior |
-|---------------|------|----------|
-| Fix files + test files | Full verification | FAIL without fix, PASS with fix |
-| Only test files | Verify failure only | Tests must FAIL (reproduce bug) |
+**Note:** `-RequireFullVerification` ensures the script errors if no fix files are detected, preventing silent fallback to failure-only mode.
 
-**Fix files** = any changed file NOT in test directories
-**Test files** = files in `TestCases.*` directories
+## Requirements
+
+**Verify Failure Only Mode:**
+- Test files in the PR (or working directory)
+
+**Full Verification Mode:**
+- Test files in the PR
+- Fix files in the PR (non-test code changes)
+
+The script auto-detects which mode to use based on whether fix files are present.
 
 ## Expected Output
 
-**Full mode (fix files detected):**
+**Verify Failure Only Mode:**
 ```
 ╔═══════════════════════════════════════════════════════════╗
 ║              VERIFICATION PASSED ✅                       ║
 ╠═══════════════════════════════════════════════════════════╣
-║  - FAIL without fix (as expected)                         ║
-║  - PASS with fix (as expected)                            ║
+║  Tests FAILED as expected!                                ║
+║  This proves the tests correctly reproduce the bug.       ║
 ╚═══════════════════════════════════════════════════════════╝
 ```
 
-**Verify failure only (no fix files):**
+**Full Verification Mode:**
 ```
 ╔═══════════════════════════════════════════════════════════╗
-║         VERIFICATION PASSED ✅                            ║
+║              VERIFICATION PASSED ✅                       ║
 ╠═══════════════════════════════════════════════════════════╣
-║  Tests FAILED as expected (bug is reproduced)             ║
+║  - FAIL without fix (as expected)                         ║
+║  - PASS with fix (as expected)                            ║
 ╚═══════════════════════════════════════════════════════════╝
 ```
 
+## What It Does
+
+**Verify Failure Only Mode (no fix files):**
+1. Fetches base branch from origin (if available)
+2. Auto-detects test classes from changed test files
+3. Runs tests (should FAIL to prove they catch the bug)
+4. Reports result
+
+**Full Verification Mode (fix files detected):**
+1. Fetches base branch from origin to ensure accurate diff
+2. Auto-detects fix files (non-test code) from git diff
+3. Auto-detects test classes from `TestCases.Shared.Tests/*.cs`
+4. Reverts fix files to base branch
+5. Runs tests (should FAIL without fix)
+6. Restores fix files
+7. Runs tests (should PASS with fix)
+8. Reports result
+
 ## Troubleshooting
 
 | Problem | Cause | Solution |
 |---------|-------|----------|
+| No fix files detected | Base branch detection failed or no non-test files changed | Use `-FixFiles` or `-BaseBranch` explicitly |
 | Tests pass without fix | Tests don't detect the bug | Review test assertions, update test |
-| Tests pass (no fix files) | **Test is wrong** | Review test vs issue description, fix test |
+| Tests fail with fix | Fix doesn't work or test is wrong | Review fix implementation |
 | App crashes | Duplicate issue numbers, XAML error | Check device logs |
 | Element not found | Wrong AutomationId, app crashed | Verify IDs match |
 
-## What It Does
-
-**Full mode:**
-1. Auto-detects fix files (non-test code) from git diff
-2. Auto-detects test classes from `TestCases.Shared.Tests/*.cs`
-3. Reverts fix files to base branch
-4. Runs tests (should FAIL without fix)
-5. Restores fix files
-6. Runs tests (should PASS with fix)
-7. Reports result
-
-**Verify Failure Only mode:**
-1. Runs tests once
-2. Verifies they FAIL (bug reproduced)
-3. Reports result
-
 ## Optional Parameters
 
 ```bash
+# Require full verification (fail if no fix files detected) - recommended
+-RequireFullVerification
+
 # Explicit test filter
 -TestFilter "Issue32030|ButtonUITests"
 
 # Explicit fix files  
 -FixFiles @("src/Core/src/File.cs")
 
-# Verify failure only (no fix exists yet)
--VerifyFailureOnly
+# Explicit base branch
+-BaseBranch "main"
 ```