chore: improve metadata service error handling

[google-labs-jules]

Improved error handling in google/auth/compute_engine/_metadata.py to provide more informative error messages when retries are exhausted.

The logic now handles two distinct failure modes:

1. Connection Errors (Exceptions):

   • Exceptions caught during the request (e.g., connection refused) are stored in last_exception.
   • If the retry loop exhausts and last_exception is set, a TransportError is raised that chains the original exception. This preserves the stack trace and specific error type for debugging.

2. Retryable HTTP Status Codes:

   • If a request completes but returns a retryable status code (e.g., 500, 503), last_exception is cleared (set to None).
   • If the retry loop exhausts due to repeated retryable status codes, a TransportError is raised that includes the response.status and the response body (error_details). This ensures the specific server error message is visible.

This addresses feedback from PR 1637 by ensuring that "Ran out of retries" scenarios provide the most relevant context: either the underlying exception or the final HTTP error response.

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PR created automatically by Jules for task 6226991344715693997 started by @chalmerlowe

[google-labs-jules]

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[chalmerlowe]

LGTM