google-auth: Local developer mTLS environment leaks into unit tests, causing mock failures

Start in tests/conftest.py and review the existing pytest fixtures and mTLS-related tests. Run the unit test suite with local mTLS environment variables or certificate configuration present, then verify ordinary tests use standard endpoints while tests that explicitly enable mTLS still pass. Done means the environment is isolated per test and restored afterward.

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Summary of the issue

When running unit tests locally on developer machines that have context-aware access or enterprise mTLS configured (such as Google-managed corporate MacBooks), several tests fail unexpectedly.

This happens because the test suite's global configuration (tests/conftest.py) does not sanitize the environment. Global environment variables (like GOOGLE_API_CERTIFICATE_CONFIG or GOOGLE_API_USE_CLIENT_CERTIFICATE) and default configuration files (like ~/.config/gcloud/certificate_config.json) leak directly into the pytest session.

Any test that does not explicitly mock _mtls_helper.check_use_client_cert() or clear these environment variables will transition to using mTLS hostnames (e.g. expecting iamcredentials.googleapis.com but getting iamcredentials.mtls.googleapis.com). Since the mock networks in the tests are configured with standard endpoints, the requests fail to match the mock setups, causing transport exceptions and test failures.

Proposed fix

Introduce a function-scoped, autouse fixture in tests/conftest.py to establish a clean, hermetic environment for every unit test. Using pytest's standard monkeypatch fixture ensures all local environmental variables and mocks are automatically cleaned up and restored back to the developer's original workstation state after each test executes. This can be accomplished by something like:

# tests/conftest.py
@pytest.fixture(autouse=True)
def clean_mtls_environment(monkeypatch):
    from google.auth.transport import _mtls_helper

    # Pop all mTLS-related environment variables
    for var in [
        "GOOGLE_API_USE_CLIENT_CERTIFICATE",
        "GOOGLE_API_CERTIFICATE_CONFIG",
        "CLOUDSDK_CONTEXT_AWARE_USE_CLIENT_CERTIFICATE",
        "CLOUDSDK_CONTEXT_AWARE_CERTIFICATE_CONFIG_FILE_PATH",
    ]:
        monkeypatch.delenv(var, raising=False)

    # Mock check_use_client_cert to return False by default
    monkeypatch.setattr(_mtls_helper, "check_use_client_cert", lambda: False)

This way tests specifically designed to verify mTLS behaviors can easily override the global mock locally by re-patching the method in their own bodies (which runs after the global autouse setup):

def test_mtls_behavior(monkeypatch):
    from google.auth.transport import _mtls_helper
    monkeypatch.setattr(_mtls_helper, "check_use_client_cert", lambda: True)
    # Test runs with mTLS enabled cleanly