* fix: treat HTTP 502/503/504 as failover-eligible (timeout reason)
When a model API returns 502 Bad Gateway, 503 Service Unavailable, or
504 Gateway Timeout, the error object carries the status code directly.
resolveFailoverReasonFromError() only checked 402/429/401/403/408/400,
so 5xx server errors fell through to message-based classification which
requires the status code to appear at the start of the error message.
Many API SDKs (Google, Anthropic) set err.status = 503 without prefixing
the message with '503', so the message classifier never matched and
failover never triggered — the run retried the same broken model.
Add 502/503/504 to the status-code branch, returning 'timeout' (matching
the existing behavior of isTransientHttpError in the message classifier).
Fixes#20999
* Changelog: add failover 502/503/504 note with credits
* Failover: classify HTTP 504 as transient in message parser
* Changelog: credit taw0002 and vincentkoc for failover fix
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Co-authored-by: Vincent Koc <vincentkoc@ieee.org>
