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/DefaultValueProvider.gd

@@ -0,0 +1,13 @@
+# default value provider, simple returns the initial value
+class_name DefaultValueProvider
+extends ValueProvider
+
+var _value: Variant
+
+
+func _init(value: Variant) -> void:
+	_value = value
+
+
+func get_value() -> Variant:
+	return _value