fix(agent): set retryable=False for auth errors in _classify_by_message

[JiayuuWang]

Summary

In agent/error_classifier.py, the _classify_by_message() function (the fallback path when no HTTP status code is available) marks auth errors as retryable=True. This is inconsistent with the _classify_by_status() path which correctly uses retryable=False for 401/403 auth errors.

Fix

Change retryable=True → retryable=False in the auth pattern branch of _classify_by_message(). Auth errors indicate the credential is invalid; retrying with the same key will always fail. The should_rotate_credential=True already signals the correct recovery action.

Impact

When an API response lacks an HTTP status code but contains an auth-related message, the classifier previously returned retryable=True for auth failures. This caused:

1. The caller retries with the same invalid credential instead of rotating
2. Creates an unnecessary retry loop before eventually hitting max retries
3. Wastes time and potentially burns rate-limit quota with doomed requests

Fixing this aligns the message-based path with the status-code path behavior.

Test plan

• Run existing error classifier tests: pytest tests/agent/test_error_classifier.py -v
• Verify auth error classification returns retryable=False

Fixes #7026.

[teknium1]

Closed in favor of #7027 which includes the same fix plus an explanatory inline comment. Your fix was identical and correct — thanks for the contribution, @JiayuuWang!