google-auth: Tests in test__default.py fail if GOOGLE_CLOUD_PROJECT is set in the local environment #17759

If you run the test suite locally with GOOGLE_CLOUD_PROJECT set in your environment, several tests in tests/test__default.py (such as test_default_early_out and test_default_gdch_service_account_credentials) will fail.

The google.auth.default() function reads os.environ.get("GOOGLE_CLOUD_PROJECT") to determine the project ID. Because os.environ isn't fully mocked or cleared in these tests, the local environment variable takes precedence over the mocked project_id sentinels, resulting in assertion failures.

To fix this, we should properly isolate the tests from the local environment by clearing or mocking the project-related environment variables (e.g., using pytest's monkeypatch.delenv) inside test__default.py.