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.

project.godot

@@ -43,7 +43,7 @@ project/assembly_name="MusicPlayer"
 
 [editor_plugins]
 
-enabled=PackedStringArray("res://addons/kenyoni/qr_code/plugin.cfg")
+enabled=PackedStringArray("res://addons/kenyoni/qr_code/plugin.cfg", "res://addons/gdUnit4/plugin.cfg")
 
 [input]