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.

tests/mocks/mock_http_request.gd

@@ -0,0 +1,37 @@
+class_name MockHTTPRequest
+extends HTTPRequest
+
+signal mock_request_completed(result: int, response_code: int, headers: PackedStringArray, body: PackedByteArray)
+
+var mock_responses: Dictionary = {}
+var request_count: int = 0
+
+func _ready() -> void:
+	request_completed.connect(_on_request_completed)
+
+func _on_request_completed(result: int, response_code: int, headers: PackedStringArray, body: PackedByteArray) -> void:
+	pass
+
+func add_mock_response(url: String, response_code: int = 200, body: PackedByteArray = PackedByteArray(), headers: PackedStringArray = PackedStringArray()) -> void:
+	mock_responses[url] = {
+		"code": response_code,
+		"body": body,
+		"headers": headers
+	}
+
+func request(url: String, headers: PackedStringArray = [], method: int = HTTPClient.METHOD_GET, body: PackedByteArray = PackedByteArray()) -> int:
+	request_count += 1
+	if mock_responses.has(url):
+		var response = mock_responses[url]
+		emit_signal("request_completed", OK, response.code, response.headers, response.body)
+		return OK
+	else:
+		emit_signal("request_completed", FAILED, 404, [], PackedByteArray())
+		return FAILED
+
+func get_request_count() -> int:
+	return request_count
+
+func reset() -> void:
+	request_count = 0
+	mock_responses.clear()

tests/mocks/test_helpers.gd

@@ -0,0 +1,23 @@
+class_name TestHelpers
+extends RefCounted
+
+static func create_mock_audio_stream() -> AudioStreamMP3:
+	var stream = AudioStreamMP3.new()
+	stream.data = PackedByteArray()
+	return stream
+
+static func create_test_song_object(song_no: int = 1, soundtrack: String = "Test Soundtrack", song_title: String = "Test Song") -> SongObject:
+	var song_obj = SongObject.new()
+	song_obj.song_number = song_no
+	song_obj.soundtrack_title = soundtrack
+	song_obj.song_title = song_title
+	song_obj.song = create_mock_audio_stream()
+	return song_obj
+
+static func setup_test_environment() -> void:
+	Settings.is_debug = true
+	Settings.stop_after_current = false
+	Settings.auto_repeat_song = false
+
+static func cleanup_test_environment() -> void:
+	Settings.is_debug = false