fix: Mock google.auth.default in test_fast_api.py

Patch google.auth.default to return a mock credential and a test project ID, ensuring tests don't rely on real authentication.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 928943169

Author: DeanChensj

tests/unittests/cli/test_fast_api.py

@@ -957,6 +957,7 @@ async def stream_query_impl(**kwargs):
   mock_adk_app_instance.stream_query = stream_query_impl
 
   with (
+      patch("google.auth.default", return_value=(MagicMock(), "test-project")),
       patch("vertexai.init", new_callable=MagicMock) as mock_vertexai_init,
       patch(
           "vertexai.agent_engines.AdkApp", return_value=mock_adk_app_instance