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.

.gitignore

@@ -6,3 +6,15 @@
 .idea
 .vscode
 .DS_Store
+
+# Test artifacts
+test_results.*
+coverage/
+.gitea/
+
+# GdUnit4
+addons/gdUnit4/
+
+# Local test data
+*.gdunit
+*.gdunit.*