ci: exclude integration tests from CI

Integration tests depend on an external Phoenix server and should not
block CI when the server is temporarily unreachable. They can still be
run locally or manually with: dotnet test --filter "Category=Integration"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Mazyod

.github/workflows/dotnet.yml

@@ -31,7 +31,7 @@ jobs:
     - name: Build
       run: dotnet build --no-restore
     - name: Test
-      run: dotnet test --no-build --verbosity normal --collect:"XPlat Code Coverage"
+      run: dotnet test --no-build --verbosity normal --collect:"XPlat Code Coverage" --filter "Category!=Integration"
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@v5
       with: