Add GdUnit4 testing framework with comprehensive test coverage

- Add GdUnit4 v6.2.1 framework in addons/gdUnit4/
- Create test directory structure (tests/unit/, tests/integration/, tests/mocks/)
- Add unit tests for MusicPlayer.gd (8 tests)
- Add unit tests for MainWindow.gd (15+ tests)
- Add unit tests for Settings.gd (5 tests)
- Add unit tests for Playlist.gd (11 tests)
- Add integration tests for full flow (10+ tests)
- Add MockHTTPRequest for HTTP request mocking
- Add TestHelpers with utility functions
- Add Gitea Actions CI workflow (.gitea/workflows/test.yml)
- Add .runsettings for GdUnit4 CLI configuration
- Add run_tests.sh for convenient test execution
- Add TESTING.md with comprehensive documentation
- Add TESTING_PLAN.md with implementation details
- Update .gitignore for test artifacts
- Enable GdUnit4 plugin in project.godot

Features:
- Test button functionality (play, pause, search, sync, etc.)
- Mock REST API requests to server
- Test complete application flows
- Run tests in CI (headless mode)
- Run tests in GUI for visual feedback
- JUnit XML and HTML test reports

Generated by OpenCode AI assistant.

.gitea/workflows/test.yml

@@ -0,0 +1,62 @@
+name: Run Tests
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+jobs:
+  test:
+    name: Run GdUnit4 Tests
+    runs-on: ubuntu-latest
+    
+    steps:
+      - name: Checkout Repository
+        uses: https://github.com/actions/checkout@v4
+
+      - name: Setup Godot 4.7
+        uses: https://github.com/godotengine/godot-action@v1
+        with:
+          godot-version: 4.7
+
+      - name: Install GdUnit4
+        run: |
+          # GdUnit4 is already included in the repo at addons/gdUnit4
+          # Just verify it exists
+          if [ ! -d "addons/gdUnit4" ]; then
+            echo "GdUnit4 not found, downloading..."
+            wget https://github.com/godot-gdunit-labs/gdUnit4/releases/download/v6.2.1/gdUnit4-v6.2.1.zip
+            unzip -q gdUnit4-v6.2.1.zip
+            mv gdUnit4-v6.2.1/addons/gdUnit4 addons/gdUnit4
+            rm -rf gdUnit4-v6.2.1 gdUnit4-v6.2.1.zip
+          fi
+
+      - name: Run Tests (Headless)
+        run: |
+          # Run tests in headless mode
+          godot --path . -s addons/gdUnit4/test.gd --gdunit.cli
+        env:
+          GODOT_HEADLESS: 1
+
+      - name: Upload Test Results
+        if: always()
+        uses: https://github.com/actions/upload-artifact@v3
+        with:
+          name: test-results
+          path: |
+            test_results.xml
+            test_results.html
+          retention-days: 30
+
+      - name: Fail if Tests Failed
+        run: |
+          if [ ! -f "test_results.xml" ]; then
+            echo "Test results not found!"
+            exit 1
+          fi
+          # Check if there were any failures in the JUnit XML
+          if grep -q '<failure' test_results.xml; then
+            echo "Tests failed!"
+            exit 1
+          fi