fix: treat HTTP 502/503/504 as failover-eligible (timeout reason) (#21017)

* 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

---------

Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/agents/pi-embedded-helpers/errors.ts

@@ -120,7 +120,7 @@ const HTTP_STATUS_PREFIX_RE = /^(?:http\s*)?(\d{3})\s+(.+)$/i;
 const HTTP_STATUS_CODE_PREFIX_RE = /^(?:http\s*)?(\d{3})(?:\s+([\s\S]+))?$/i;
 const HTML_ERROR_PREFIX_RE = /^\s*(?:<!doctype\s+html\b|<html\b)/i;
 const CLOUDFLARE_HTML_ERROR_CODES = new Set([521, 522, 523, 524, 525, 526, 530]);
-const TRANSIENT_HTTP_ERROR_CODES = new Set([500, 502, 503, 521, 522, 523, 524, 529]);
+const TRANSIENT_HTTP_ERROR_CODES = new Set([500, 502, 503, 504, 521, 522, 523, 524, 529]);
 const HTTP_ERROR_HINTS = [
   "error",
   "bad request",