Add coverage report merging for multi-framework support

Uses ReportGenerator to merge coverage reports from .NET 8, 9, and 10
before uploading to Codecov. This ensures the #else fallback branch
(only compiled in .NET 8/9) shows as covered in the final report.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ANcpLua

.github/workflows/tests.yml

@@ -36,9 +36,19 @@ jobs:
             --collect:"XPlat Code Coverage" \
             --logger:"console;verbosity=detailed"
 
+      - name: Install ReportGenerator
+        run: dotnet tool install --global dotnet-reportgenerator-globaltool
+
+      - name: Merge coverage reports
+        run: |
+          reportgenerator \
+            -reports:"**/coverage.cobertura.xml" \
+            -targetdir:"./coverage-merged" \
+            -reporttypes:"Cobertura"
+
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v5
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
-          files: ./SWEN3.Paperless.RabbitMq.Tests/TestResults/**/coverage.cobertura.xml
+          files: ./coverage-merged/Cobertura.xml
           fail_ci_if_error: false