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/test/mocker/resources/CustomClassExtendsCustomClass.gd

@@ -0,0 +1,10 @@
+class_name CustomClassExtendsCustomClass
+extends CustomResourceTestClass
+
+# override
+@warning_ignore("untyped_declaration")
+func foo2():
+	return "foo2 overriden"
+
+func bar2() -> String:
+	return bar(23, 42)