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/asserts/CallBackValueProviderTest.gd

@@ -0,0 +1,23 @@
+# GdUnit generated TestSuite
+#warning-ignore-all:unused_argument
+#warning-ignore-all:return_value_discarded
+class_name CallBackValueProviderTest
+extends GdUnitTestSuite
+
+# TestSuite generated from
+const __source = 'res://addons/gdUnit4/src/asserts/CallBackValueProvider.gd'
+
+
+func next_value() -> String:
+	return "a value"
+
+
+func test_get_value() -> void:
+	var vp := CallBackValueProvider.new(self, "next_value")
+	assert_str(await vp.get_value()).is_equal("a value")
+
+
+func test_construct_invalid() -> void:
+	var vp := CallBackValueProvider.new(self, "invalid_func", Array(), false)
+	# will return null because of invalid function name
+	assert_str(await vp.get_value()).is_null()