Integrate trx-to-vsplaylist in CI test pipeline (#376)

Adds test playlist generation to enable local re-execution of failed
tests in Visual Studio.

## Changes

- Modified `dotnet test` step to output TRX files (`--logger trx
--results-directory ./TestResults`)
- Added `trx-to-vsplaylist@v2` action to generate failed tests playlist
- Added `trx-to-vsplaylist@v2` action to generate all tests playlist

Both playlist steps run with `if: always()` to ensure artifact creation
even on test failure. Playlists are uploaded as workflow artifacts for
download.

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<details>

<summary>Original prompt</summary>

> Add https://github.com/BenjaminMichaelis/trx-to-vsplaylist usage in
our test pipeline


</details>



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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>
Co-authored-by: Benjamin Michaelis <git@relay.benjamin.michaelis.net>

Author: Copilot

.github/workflows/dotnetBuild.yml

@@ -43,4 +43,12 @@ jobs:
       run: dotnet build -p:ContinuousIntegrationBuild=True --no-restore --configuration Release
 
     - name: Test
-      run: dotnet test --no-build --configuration Release --verbosity normal
+      run: dotnet test --no-build --configuration Release --verbosity normal --logger trx --results-directory ./TestResults
+    
+    - name: Create Failed Tests Playlist
+      if: always()
+      uses: BenjaminMichaelis/trx-to-vsplaylist@v2
+      with:
+        trx-file-path: './TestResults/*.trx'
+        test-outcomes: 'Failed'
+        artifact-name: 'failed-tests-playlist'