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/ClassWithVariantDefaultArguments.gd

@@ -0,0 +1,9 @@
+class_name ClassWithVariantDefaultArguments
+extends Resource
+
+@warning_ignore("unused_parameter")
+func add_button(
+	ucids: Array[int], position: Vector2i, size: Vector2i, style: int, text: Variant = "",
+	button_name := "", type_in := 0, caption := "", show_everywhere := false
+) -> void:
+	pass