chore(client): add gdUnit4 test framework addon

Vendor gdUnit4 for Godot 4 client testing (D-030).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/addons/gdUnit4/src/asserts/CallBackValueProvider.gd

@@ -0,0 +1,25 @@
+# a value provider unsing a callback to get `next` value from a certain function
+class_name CallBackValueProvider
+extends ValueProvider
+
+var _cb :Callable
+var _args :Array
+
+
+func _init(instance :Object, func_name :String, args :Array = Array(), force_error := true) -> void:
+	_cb = Callable(instance, func_name);
+	_args = args
+	if force_error and not _cb.is_valid():
+		push_error("Can't find function '%s' checked instance %s" % [func_name, instance])
+
+
+func get_value() -> Variant:
+	if not _cb.is_valid():
+		return null
+	if _args.is_empty():
+		return await _cb.call()
+	return await _cb.callv(_args)
+
+
+func dispose() -> void:
+	_cb = Callable()