chore: improve metadata service error handling

Distinguish between retry exhaustion due to exceptions and retry exhaustion due to retryable status codes.
- Use `last_exception` to track the most recent exception.
- Clear `last_exception` on successful response (even if status is retryable).
- Raise TransportError chained from `last_exception` if available.
- Otherwise raise TransportError with response status and details.
- Update tests to match new error format.

Author: google-labs-jules[bot]