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/ValueProvider.gd

@@ -0,0 +1,10 @@
+# base interface for assert value provider
+class_name ValueProvider
+extends RefCounted
+
+func get_value() -> Variant:
+	return null
+
+
+func dispose() -> void:
+	pass