resolve_xai_oauth_runtime_credentials() called _refresh_xai_oauth_tokens()
with no try/except. A terminal refresh failure (HTTP 400/401/403 —
invalid_grant, token revoked) propagated without clearing the dead
access_token / refresh_token from auth.json, causing every subsequent
session to retry the same doomed network request.
Add a try/except around the refresh call that mirrors the existing
credential_pool.py quarantine: when _is_terminal_xai_oauth_refresh_error
identifies a non-retryable failure, clear the dead token fields from
auth.json and write a last_auth_error diagnostic marker so future calls
fail fast with a clear relogin_required error instead of hitting the
network.
active_provider is preserved (set_active=False) so multi-provider users
whose chosen provider is not xai-oauth are unaffected.
Tests: two new cases in test_auth_xai_oauth_provider.py cover terminal
quarantine and transient pass-through.
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