Enforce exactly one Category attribute per UI test (#33242)

## Description

This PR enforces that every UI test has **exactly one** `[Category]`
attribute through a build-time analyzer. Previously, tests could have
multiple categories which caused them to run multiple times in CI.

Each test needs to have a category so they are ran in the CI pipeline.
With this analyzer we ensure that this is not forgotten during
development.

## Changes

### Analyzer Updates (`NUnitTestMissingCategoryAnalyzer`)

- **MAUI0001**: Build error when a test has **no** category
- **MAUI0002**: Build error when a test has **more than one** category
- Changed severity from `Warning` to `Error`
- Excludes platform-specific ignore attributes
(`FailsOnAndroidWhenRunningOnXamarinUITest`, etc.) from the category
count since they conditionally derive from `CategoryAttribute`

### Fixed 314 Tests

All tests with multiple categories have been fixed by keeping the most
appropriate single category:

- **Control-specific categories** preferred (Button, Entry,
CollectionView, ListView, etc.)
- **Feature categories** kept when no control-specific category applied
(Navigation, Shell, Gestures, etc.)
- **Removed generic categories** like `Compatibility` in favor of more
specific ones

### Added Analyzer Unit Tests

New test project `UITest.Analyzers.Tests` with 20 tests covering:
- Missing category detection
- Multiple categories detection
- Custom `CategoryAttribute` derivatives
- Platform ignore attribute exclusions
- Edge cases

## CI Impact

Tests with multiple categories were running multiple times because the
CI uses category-based test filtering with OR logic. For example, a test
with `[Category("Button")]` and `[Category("Compatibility")]` would run
in both the Button job and the Compatibility job.

**Before this PR:**
- ~314 tests had multiple categories
- Each ran 2-3x per platform depending on category count
- Estimated **~1,256 duplicate test executions** per CI run (314 × ~1
extra × 4 platforms)

**After this PR:**
- Every test runs exactly once per platform
- Build fails if a developer adds multiple categories
- Significant CI time savings

## Testing

- [x] All 314 affected tests updated
- [x] Test project builds successfully with 0 errors
- [x] Analyzer unit tests pass (20/20)
- [x] Verified analyzer catches both missing and multiple category
violations

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: rmarinho <[email protected]>
Co-authored-by: Shane Neuville <[email protected]>

Author: 4 people

Microsoft.Maui-dev.sln

@@ -337,7 +337,6 @@ public void GraphicsView_EndInteraction_VerifyEventTriggered()
 
 	[Test]
 	[Category(UITestCategories.GraphicsView)]
-	[Category(UITestCategories.Gestures)]
 	public void GraphicsView_DragInteraction_VerifyEventTriggered()
 	{
 		App.WaitForElement("ClearEventsButton");

src/Controls/tests/TestCases.Shared.Tests/Tests/FeatureMatrix/GraphicsViewFeatureTests.cs

@@ -14,7 +14,6 @@ public Bugzilla23942(TestDevice testDevice) : base(testDevice)
 
 		[Test]
 		[Category(UITestCategories.LifeCycle)]
-		[Category(UITestCategories.Compatibility)]
 		public void Bugzilla23942Test()
 		{
 			App.WaitForElement("success");

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Bugzilla/Bugzilla23942.cs

@@ -19,7 +19,6 @@ public Bugzilla25943(TestDevice testDevice) : base(testDevice)
 
 		[Test]
 		[Category(UITestCategories.Gestures)]
-		[Category(UITestCategories.Compatibility)]
 		public void VerifyNestedStacklayoutTapsBubble()
 		{
 			App.WaitForElement(InnerLayout);

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Bugzilla/Bugzilla25943.cs

@@ -6,7 +6,6 @@
 namespace Microsoft.Maui.TestCases.Tests.Issues
 {
 	[Category(UITestCategories.ScrollView)]
-	[Category(UITestCategories.Compatibility)]
 	public class Bugzilla28570UITests : _IssuesUITest
 	{
 		public Bugzilla28570UITests(TestDevice device)

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Bugzilla/Bugzilla28570.cs

@@ -15,8 +15,7 @@ public Bugzilla29128(TestDevice testDevice) : base(testDevice)
 		public override string Issue => "Slider background lays out wrong Android";
 
 		[Test]
-		[Category(UITestCategories.LifeCycle)]
-		[Category(UITestCategories.Compatibility)]
+		[Category(UITestCategories.Slider)]
 		public void Bugzilla29128Test()
 		{
 			App.WaitForElement("SliderId");

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Bugzilla/Bugzilla29128.cs

@@ -14,7 +14,6 @@ public Bugzilla29363(TestDevice testDevice) : base(testDevice)
 
 		[Test]
 		[Category(UITestCategories.Navigation)]
-		[Category(UITestCategories.Compatibility)]
 		public void PushButton()
 		{
 			App.WaitForElement("ModalPushPopTest");

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Bugzilla/Bugzilla29363.cs

@@ -14,7 +14,6 @@ public Bugzilla29453(TestDevice testDevice) : base(testDevice)
 
 		[Test]
 		[Category(UITestCategories.Navigation)]
-		[Category(UITestCategories.Compatibility)]
 		public void Bugzilla29453Test()
 		{
 			App.WaitForElement("Page1");

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Bugzilla/Bugzilla29453.cs

@@ -14,7 +14,6 @@ public Bugzilla30166(TestDevice testDevice) : base(testDevice)
 
 		[Test]
 		[Category(UITestCategories.Navigation)]
-		[Category(UITestCategories.Compatibility)]
 		public void Bugzilla30166Test()
 		{
 			App.WaitForElement("PushModal");

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Bugzilla/Bugzilla30166.cs

@@ -14,7 +14,6 @@ public Bugzilla30935(TestDevice testDevice) : base(testDevice)
 
 		[Test]
 		[Category(UITestCategories.Page)]
-		[Category(UITestCategories.Compatibility)]
 		public void Bugzilla30935DoesntThrowException()
 		{
 			App.WaitForNoElement("IssuePageLabel");