A GPT-5 model rejecting max_tokens returns a 400 whose message contains the
literal substring 'max_tokens' — one of the _CONTEXT_OVERFLOW_PATTERNS. The 400
path in _classify_400 checked overflow patterns before any request-validation
check (which only existed on the 5xx path), so the parameter error was routed
into the compression loop, re-sent with the same bad param, and ended in
'Cannot compress further' on a tiny context.
Hoist a request-validation guard (unsupported/unknown parameter) above the
context-overflow check in _classify_400. Deliberately excludes the generic
invalid_request_error code, which OpenAI also stamps on real overflow 400s, so
genuine overflows still compress. Pairs with the max_completion_tokens param
fix that stops the bad request at the source.
Also adds AUTHOR_MAP entry for the salvaged PR #13902 commit.
2ce3ae3d16