xAI's Responses stream emits 'type=error' as the FIRST SSE frame when an
OAuth account is unsubscribed/exhausted or rejects the encrypted-reasoning
replay introduced in the May 2026 SuperGrok rollout. The SDK helper
raises RuntimeError(Expected to have received response.created before
error), which the caller correctly routes to
_run_codex_create_stream_fallback. The fallback then opens a new stream
that emits the same 'error' frame — but the fallback loop only handled
{response.completed, response.incomplete, response.failed} and silently
continue'd past 'error' events. Result: the loop fell off the end of
the stream and raised the useless 'fallback did not emit a terminal
response' RuntimeError, which the classifier marked retryable=True and
looped 3x before failing with no clue what went wrong.
Now: 'error' frames raise a synthesized _StreamErrorEvent with an OpenAI
SDK-shaped .body so _summarize_api_error, _extract_api_error_context,
_is_entitlement_failure, and classify_api_error all see the real
provider message. Users on unsubscribed accounts now see 'do not have
an active Grok subscription' once, not three RuntimeErrors.
Verified end-to-end: classifier returns reason=auth retryable=False;
entitlement detector matches even with status_code=None; summarizer
returns the full xAI message.
Tests: 4 new in TestCodexFallbackErrorEvent covering xAI subscription
message, dict-shaped events, summarizer integration, and the empty-stream
case (must still raise the original RuntimeError so 'truncated mid-flight'
stays distinguishable from 'provider rejected the call').
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