Replace the broken test suite with working GdUnit4 tests

The previous tests never compiled: they used GUT's assert functions,
Godot 3 APIs and reassigned autoload functions, so no test ran.

- 90 tests: unit tests for the playlist, songs, player scoring, time
  formatting, search and the server client, plus integration tests
  that run MainWindow and SyncWindow against a fake server
- Keybind tests check that Alt+<number> and Ctrl+Alt+<number> keep
  following the player rows while players are added and removed
- run_tests.sh finds Godot and runs the suite headless (~10 seconds)
- CI runs the tests before the build job in gitea-build.yaml; the
  separate test.yml used a GitHub action that does not exist
- GdUnit4 is committed instead of ignored, since the project enables
  it, and .gitea/ is no longer ignored
- Exported builds leave out tests/ and addons/gdUnit4/

addons/gdUnit4/src/dotnet/GdUnit4CSharpApiLoader.gd

@@ -0,0 +1,114 @@
+## GdUnit4CSharpApiLoader
+##
+## A bridge class that handles communication between GDScript and C# for the GdUnit4 testing framework.
+## This loader acts as a compatibility layer to safely access the .NET API and ensure that calls
+## only proceed when the .NET environment is properly configured and available.
+## [br]
+## The class handles:
+## - Verification of .NET runtime availability
+## - Loading the C# wrapper script
+## - Checking for the GdUnit4Api assembly
+## - Providing proxy methods to access GdUnit4 functionality in C#
+@static_unload
+class_name GdUnit4CSharpApiLoader
+extends RefCounted
+
+## Cached reference to the loaded C# wrapper script
+static var _gdUnit4NetWrapper: Script
+
+## Cached instance of the API (singleton pattern)
+static var _api_instance: RefCounted
+
+
+class TestEventListener extends RefCounted:
+
+	func publish_event(event: Dictionary) -> void:
+		var test_event := GdUnitEvent.new().deserialize(event)
+		GdUnitSignals.instance().gdunit_event.emit(test_event)
+
+static var _test_event_listener := TestEventListener.new()
+
+
+## Returns an instance of the GdUnit4CSharpApi wrapper.[br]
+## @return Script: The loaded C# wrapper or null if .NET is not supported
+static func instance() -> Script:
+	if not GdUnit4CSharpApiLoader.is_api_loaded():
+		return null
+
+	return _gdUnit4NetWrapper
+
+
+## Returns or creates a single instance of the API [br]
+## This improves performance by reusing the same object
+static func api_instance() -> RefCounted:
+	if _api_instance == null and is_api_loaded():
+		@warning_ignore("unsafe_method_access")
+		_api_instance = instance().new()
+	return _api_instance
+
+
+static func is_engine_version_supported(engine_version: int = Engine.get_version_info().hex) -> bool:
+	return engine_version >= 0x40200
+
+
+## Checks if the .NET environment is properly configured and available.[br]
+## @return bool: True if .NET is fully supported and the assembly is found
+static func is_api_loaded() -> bool:
+	# If the wrapper is already loaded we don't need to check again
+	if _gdUnit4NetWrapper != null:
+		return true
+
+	# First we check if this is a Godot .NET runtime instance
+	if not ClassDB.class_exists("CSharpScript") or not is_engine_version_supported():
+		return false
+	# Second we check the C# project file exists
+	var assembly_name: String = ProjectSettings.get_setting("dotnet/project/assembly_name")
+	if assembly_name.is_empty() or not FileAccess.file_exists("res://%s.csproj" % assembly_name):
+		return false
+
+	# Finally load the wrapper and check if the GdUnit4 assembly can be found
+	_gdUnit4NetWrapper = load("res://addons/gdUnit4/src/dotnet/GdUnit4CSharpApi.cs")
+	@warning_ignore("unsafe_method_access")
+	return _gdUnit4NetWrapper.call("IsApiLoaded")
+
+
+## Returns the version of the GdUnit4 .NET assembly.[br]
+## @return String: The version string or "unknown" if .NET is not supported
+static func version() -> String:
+	if not GdUnit4CSharpApiLoader.is_api_loaded():
+		return "unknown"
+	@warning_ignore("unsafe_method_access")
+	return instance().Version()
+
+
+static func discover_tests(source_script: Script) -> Array[GdUnitTestCase]:
+	var tests: Array = _gdUnit4NetWrapper.call("DiscoverTests", source_script)
+
+	return Array(tests.map(GdUnitTestCase.from_dict), TYPE_OBJECT, "RefCounted", GdUnitTestCase)
+
+
+static func execute(tests: Array[GdUnitTestCase]) -> void:
+	var net_api := api_instance()
+	if net_api == null:
+		push_warning("Execute C# tests not supported!")
+		return
+	var tests_as_dict: Array[Dictionary] = Array(tests.map(GdUnitTestCase.to_dict), TYPE_DICTIONARY, "", null)
+
+	net_api.call("ExecuteAsync", tests_as_dict, _test_event_listener.publish_event)
+	@warning_ignore("unsafe_property_access")
+	await net_api.ExecutionCompleted
+
+
+static func create_test_suite(source_path: String, line_number: int, test_suite_path: String) -> GdUnitResult:
+	if not GdUnit4CSharpApiLoader.is_api_loaded():
+		return  GdUnitResult.error("Can't create test suite. No .NET support found.")
+	@warning_ignore("unsafe_method_access")
+	var result: Dictionary = instance().CreateTestSuite(source_path, line_number, test_suite_path)
+	if result.has("error"):
+		return GdUnitResult.error(str(result.get("error")))
+	return  GdUnitResult.success(result)
+
+
+static func is_csharp_file(resource_path: String) -> bool:
+	var ext := resource_path.get_extension()
+	return ext == "cs" and GdUnit4CSharpApiLoader.is_api_loaded()