fix(retry): retry google-auth TransportError with HttpRetryOptions

[emileferreira]

Fixes #2869

Summary

• retry google.auth.exceptions.TransportError when HttpRetryOptions is enabled
• keep permanent authentication failures such as RefreshError non-retryable
• add an async regression test proving credential refresh can recover before
  dispatching exactly one model HTTP request

Google Auth wraps token-endpoint connection failures as TransportError, so
they bypass the existing HTTPX exception branch in retry_args(). Credential
acquisition occurs inside _async_request_once() and before model dispatch,
which means the existing Tenacity boundary can safely retry this failure.

This follows the precedent established by #2337 and #2345 for lower-level
HTTPX transport errors. It also matches the retry predicates used by other
Google Python clients such as Cloud Storage and BigQuery.

Retries remain opt-in through HttpRetryOptions. This change does not retry
RefreshError, invalid grants, missing credentials, or other permanent
authentication failures.

Testing

• pytest google/genai/tests/client/test_retries.py::test_retry_args_retries_transport_errors google/genai/tests/client/test_retries.py::test_async_retries_google_auth_transport_error -q
  • 2 passed
• mypy google/genai/
  • success
• git diff --check
  • success

Scope note

The aiohttp branches already perform one hard-coded retry for
TransportError. This PR leaves that existing no-options behaviour unchanged;
consolidating the aiohttp retry path would be a separate behavioural change.

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[Venkaiahbabuneelam]

Hi @emileferreira, Thanks for reaching out us!

Please resolve all conflict checks.
Thanks