Upload TestCases folder as artifact when CI tests fail

Failed decompiler-test runs leave their generated fixtures (compiled
test assemblies, generated IL, diff inputs) on the runner where they
are lost, making CI-only failures hard to diagnose. Capture the folder
via upload-artifact's own client-side zip at compression-level 9
instead of a separate 7z step, since 7z is not reliably available on
the macOS runner.

Assisted-by: Claude:claude-fable-5:Claude Code

.github/workflows/build-ilspy.yml

@@ -90,6 +90,7 @@ jobs:
       run: dotnet-format whitespace --verify-no-changes --verbosity detailed ILSpy.sln
 
     - name: Execute unit tests
+      id: unit-tests
       run: >
         dotnet test --solution ilspy.sln
         --configuration ${{ matrix.configuration }}
@@ -103,6 +104,14 @@ jobs:
         name: test-results-${{ matrix.configuration }}
         path: 'test-results/${{ matrix.configuration }}/*.trx'
 
+    - name: Upload TestCases on test failure
+      uses: actions/upload-artifact@v7
+      if: failure() && steps.unit-tests.outcome == 'failure'
+      with:
+        name: testcases-${{ matrix.configuration }}
+        path: ICSharpCode.Decompiler.Tests/TestCases
+        compression-level: 9
+
     - name: Create Test Report
       uses: test-summary/action@v2
       if: always()
@@ -324,6 +333,7 @@ jobs:
         --results-directory test-results
 
     - name: Execute decompiler tests
+      id: decompiler-tests
       run: >
         dotnet test --project ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj
         --configuration Release
@@ -337,6 +347,14 @@ jobs:
         name: test-results-${{ matrix.platform }}
         path: 'test-results/*.trx'
 
+    - name: Upload TestCases on test failure
+      uses: actions/upload-artifact@v7
+      if: failure() && steps.decompiler-tests.outcome == 'failure'
+      with:
+        name: testcases-${{ matrix.platform }}
+        path: ICSharpCode.Decompiler.Tests/TestCases
+        compression-level: 9
+
     - name: Create Test Report
       uses: test-summary/action@v2
       if: always()