fix(sse): keep HTTP status and retry hint in post-keepalive /v1/responses error frames

[QuangBlue]

⚠️ base-red inherited: #14547

Symptom

/v1/responses streaming requests that go through the early keepalive (#13431) are committed to
HTTP 200 after DEFAULT_THRESHOLD_MS. When the handler then returns an error response (for
example a 403 "model not allowed for this key", a 429 with Retry-After, or a 503), the client
receives a single {"type":"error", code, message, param, sequence_number} frame. The HTTP
status, the Retry-After hint and error.type are gone, so a Responses client cannot tell a
permanent failure from a transient one. In practice clients retried permanent 403s and could not
back off correctly on 429/503.

Root cause

open-sse/utils/earlyStreamKeepalive.ts, non-SSE branch of the slow path: the handler's error
Response is read as text and reshaped by buildResponsesErrorDataLine(text), which only keeps
code, message, param (plus diagnostics). The status and headers of that Response are
never looked at.

Fix

The post-commit Responses error frame now also carries:

• status_code: the handler response status (always present in this branch).
• error_type: error.type from the body, when present.
• retry_after_seconds: Retry-After (delta-seconds or HTTP-date), else
  x-omniroute-retry-after-seconds, clamped to 0..3600; omitted when neither header is usable.

Design notes:

• error_type, not type: top-level type is the Responses event discriminator.
• Flat fields, not a nested error object: openai-node throws on an event with a top-level
  error key instead of yielding it, which would undo fix(api): frame post-keepalive /v1/responses stream errors with a type field #13431.
• Chat Completions and Anthropic frames, and the fast path (response before the threshold), are
  unchanged. Only additive fields; existing keys keep their values.

Follow-up of #13431; related #14377.

Tests

• New: tests/unit/responses-post-keepalive-error-frame-status.test.ts (red before the fix,
  green after): 429 with Retry-After delta-seconds, HTTP-date Retry-After clamped to 3600,
  the x-omniroute-retry-after-seconds fallback, 403 without a retry hint, a body without
  error.type, and unchanged fast path and chat frames.
• node --import tsx/esm --test on the new test plus
  issue-13431-responses-post-keepalive-error-frame, early-stream-keepalive,
  earlyStreamKeepalive, responses-route-early-keepalive-wiring, keepalive-threshold:
  54 tests, 52 pass, 2 pre-existing skips, 0 fail.
• eslint (with the repo suppressions) clean on both files.

[diegosouzapw]

Nice fix — the design notes on error_type vs type and flat fields (avoiding what would undo
#13431) show real care. Heads up: this PR is now CONFLICTING against the release branch
because #14572 (merged a couple hours ago) touched the exact same return statement in
buildResponsesErrorDataLine to add a real sequence_number. Both changes are compatible (no
logical overlap), so it should be a quick rebase — could you pull latest release/v3.8.51 and
resolve? Happy to re-verify once that's done.

[QuangBlue]

Thanks! This one is already rebased onto the latest release/v3.8.51 (head ab0353ab9, on top of #14572): the error frame keeps sequence_number: SYNTHETIC_RESPONSES_SEQUENCE_NUMBER from #14572 and adds the status fields after it. GitHub now reports it as mergeable.