* fix: treat HTTP 503 as failover-eligible for LLM provider errors
When LLM SDKs wrap 503 responses, the leading "503" prefix is lost
(e.g. Google Gemini returns "high demand" / "UNAVAILABLE" without a
numeric prefix). The existing isTransientHttpError only matches
messages starting with "503 ...", so these wrapped errors silently
skip failover — no profile rotation, no model fallback.
This patch closes that gap:
- resolveFailoverReasonFromError: map HTTP status 503 → rate_limit
(covers structured error objects with a status field)
- ERROR_PATTERNS.overloaded: add /\b503\b/, "service unavailable",
"high demand" (covers message-only classification when the leading
status prefix is absent)
Existing isTransientHttpError behavior is unchanged; these additions
are complementary and only fire for errors that previously fell
through unclassified.
* fix: address review feedback — drop /\b503\b/ pattern, add test coverage
- Remove `/\b503\b/` from ERROR_PATTERNS.overloaded to resolve the
semantic inconsistency noted by reviewers: `isTransientHttpError`
already handles messages prefixed with "503" (→ "timeout"), so a
redundant overloaded pattern would classify the same class of errors
differently depending on message formatting.
- Keep "service unavailable" and "high demand" patterns — these are the
real gap-fillers for SDK-rewritten messages that lack a numeric prefix.
- Add test case for JSON-wrapped 503 error body containing "overloaded"
to strengthen coverage.
* fix: unify 503 classification — status 503 → timeout (consistent with isTransientHttpError)
resolveFailoverReasonFromError previously mapped status 503 → "rate_limit",
while the string-based isTransientHttpError mapped "503 ..." → "timeout".
Align both paths: structured {status: 503} now also returns "timeout",
matching the existing transient-error convention. Both reasons are
failover-eligible, so runtime behavior is unchanged.
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Co-authored-by: Vincent Koc <vincentkoc@ieee.org>
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