hero-scenarios: fall back to add-comment when file not in PR diff (#42334)

When hero-scenarios.md exists in the repo but isn't part of the PR's
changed files, the agent was using create-pull-request-review-comment
which fails with 'Path could not be resolved'. Now the prompt instructs
the agent to check whether the file is in the PR diff first and fall
back to add-comment when it isn't.

Also makes the hero-scenarios.md filename check case-insensitive to
handle existing HERO_SCENARIOS.md files.

Co-authored-by: Deyaa Eldeen <deyaaeldeen@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: deyaaeldeen

.github/prompts/hero-scenarios-guidelines.md

@@ -16,7 +16,8 @@
 
 ## Step 1 — Check for Existing Hero Scenarios
 
-Check whether the service already has a `hero-scenarios.md` file.
+Check whether the service already has a `hero-scenarios.md` file (the
+check should be case-insensitive — e.g., `HERO_SCENARIOS.md` counts).
 Look in the top-level service directory (e.g.,
 `specification/loadtestservice/hero-scenarios.md`) and in each spec subdirectory
 (e.g., `resource-manager/Microsoft.LoadTestService/loadtesting/hero-scenarios.md`,
@@ -267,19 +268,37 @@ The file should include:
    does, who it is for, and what problem it solves.
 2. **Hero scenarios** — the scenarios generated in Step 3.
 
-**If a `hero-scenarios.md` already exists**, post an inline code suggestion on
-the file using `create-pull-request-review-comment`. Place the
-comment at the end of the file and use a GitHub suggestion block so the
-author can apply it with one click:
+**If a `hero-scenarios.md` already exists**, check whether the file is
+part of the PR diff (i.e., listed in the changed files of the pull request).
 
-````
-```suggestion
-<the new scenario section as raw markdown>
-```
-````
+- **If the file IS in the PR diff**, post an inline code suggestion on
+  the file using `create-pull-request-review-comment`. Place the
+  comment at the end of the file and use a GitHub suggestion block so the
+  author can apply it with one click:
+
+  ````
+  ```suggestion
+  <the new scenario section as raw markdown>
+  ```
+  ````
+
+  Then submit the review using `submit-pull-request-review` with event
+  `COMMENT` and a one-sentence body summarizing what was suggested.
+
+- **If the file is NOT in the PR diff**, use `add-comment` to post a
+  PR comment instead (review comments can only be placed on files in the
+  diff). Format it as:
+
+  ```
+  ## 📖 Suggested additions to hero-scenarios.md
+
+  > **Path:** `specification/{service}/{specDir}/hero-scenarios.md`
+  >
+  > The existing `hero-scenarios.md` does not cover the API surface in
+  > this PR. Consider adding the following scenarios:
 
-Then submit the review using `submit-pull-request-review` with event
-`COMMENT` and a one-sentence body summarizing what was suggested.
+  <the new scenario section as raw markdown — do NOT wrap it in a code fence>
+  ```
 
 If the existing `hero-scenarios.md` already covers all the API surface in
 this PR, submit a `COMMENT` review with a one-sentence body confirming no