fix(sse): emit valid responses keepalives

[TheDemonTuan]

Summary

• Replace malformed synthetic response.in_progress heartbeat events with SSE comments
• Preserve real Responses API event sequence numbers and payloads without emitting schema-violating placeholders
• Update early- and mid-stream regression coverage

Verification

• node --import tsx/esm --test tests/unit/sseHeartbeat.test.ts tests/unit/sse-heartbeat.test.ts tests/unit/earlyStreamKeepalive.test.ts tests/unit/early-stream-keepalive.test.ts tests/unit/chatcore-streaming-pipeline.test.ts tests/unit/responses-route-early-keepalive-wiring.test.ts (all passed)

[diegosouzapw]

Re-reviewed this against the fuller bug report that has since landed (#14330), and two things need
changing before it can merge.

First, the new keepalive shape bypasses the opt-out from #10524. The gate at
open-sse/utils/sseHeartbeat.ts:107 keys on shape === HEARTBEAT_SHAPES.COMMENT, and the new
openai-responses-keepalive shape emits a : comment without being that shape — so
OMNIROUTE_SSE_COMMENTS=off no longer suppresses it, which re-breaks the strict clients that flag
was added for. Either reuse the COMMENT shape or widen the gate to any comment-shaped payload.

Second, #14330's third criterion is the invalid failure frames, and this PR doesn't touch them:
six emitters ship response.failed/error events with no sequence_number or a hardcoded 0 while
the translator numbers from 1 (streamHandler.ts:454,464-472, diagnostics.ts:143-159,
codex.ts:515-520 and :982-989, streamErrorFormat.ts:296-307,
earlyStreamKeepalive.ts:83-90,128), and the same invalid keepalive is also emitted mid-stream at
responsesHandler.ts:91-94. Either extend the PR or say in the body that #14330 stays open for
them — and please use Refs #14330 rather than Closes, since our default branch is the release
branch and Closes would auto-close it.

One coordination note: #14377 asks for the keepalive shape to be configurable, which this PR
decides one way by default. Fixing the invalid frames is unconditional and welcome; the shape choice
is probably better behind the env switch that issue asks for. Reds currently on the branch are
inherited from base-red #14496.

[diegosouzapw]

Thanks @TheDemonTuan for chasing the invalid Responses keepalive frames (#14330). You were right that the synthetic response.in_progress heartbeat did not match the Responses schema.

#14572 (merged 2026-09-24) fixed this on release/v3.8.51 in a different way. The synthesized frames stay as parser-visible events, but they now carry a real sequence_number (SYNTHETIC_RESPONSES_SEQUENCE_NUMBER in open-sse/utils/sseHeartbeat.ts and open-sse/utils/earlyStreamKeepalive.ts). We kept that design over bare : comments because some strict OpenAI-compatible clients need a periodic event frame during the early phase. This branch rewrites the same code the other way, and it also ran into the malformed-JSON admission-release regression in the responses route, so I'm closing it as superseded.

If you hit a client that still rejects the frames the tip sends now, please open an issue with a capture. We'll look at it against the current design.