Review followups for #450:

- M1: handleNonStreaming.collectErr's instanceof CodexApiError branch now
  rethrows instead of returning c.json directly, so the outer
  handleProxyRequest catch runs unified strip+retry recovery for
  mid-SSE 400s (previous_response_not_found, unanswered_function_call).
  Releases the account before the throw; outer catch deduplicates via
  the released Set.
- M2: rename prevRespNotFoundRetried → stripAndRetryDone to reflect
  that the same flag now gates two recovery paths (prev_resp_not_found
  + unanswered_function_call), single-shot to avoid retry loops.
- L2: response-processor uses instanceof CodexApiError instead of
  duck-typed body/status access — strict typing, no foreign Error
  subclasses with same field names.
- L4: drop unused TIMEOUT import from real e2e.
- L6: tighten partial-tool_result assertion — must be 4xx with the
  upstream's verbatim "No tool output found for function call"
  message; 5xx now fails the test (catches future regressions where
  the SSE/collect path loses the real upstream status).

Discovered while re-reviewing #450: handleNonStreaming was releasing the
account at L833 before falling into the new CodexApiError rethrow branch.
After rethrow, outer catch's strip+retry path continues on the same entryId
without re-acquiring — but the slot was already returned to the pool, so
another concurrent request could grab it and race the retry.

Move the unconditional release below the rethrow branch. EmptyResponseError
exhausted + generic-fallback paths still release before returning their
respective Response. The released Set continues to guard double-release on
terminal paths owned by the outer catch.

…or log

Carryover from #450 review:

- L1: evaluateImplicitResume now returns missingCallIds / unansweredCallIds
  on rejection; handleProxyRequest reads those off the result instead of
  recomputing the same set difference inline.
- L5: introduce stripCodexErrorPrefix helper to remove the "Codex API error
  (NNN): " prefix when the warn already prints status= separately, so the
  log no longer reads "upstream 400 during collect: Codex API error (400):
  No tool output ..." (duplicated status).

L3 (status-code map shared with ws-pool.ts) deferred — larger refactor,
follow-up PR.