fix(responses): use protocol-neutral keepalive before response starts

[tufw95]

Problem

The Responses endpoint emitted a synthetic rs_omniroute_keepalive reasoning item while waiting for a slow provider. That item had response.output_item.added and summary events but no response.output_item.done. Codex can therefore keep the turn in a running state (spinner/pause) after the real response emits response.completed.

Fix

Use the existing SSE comment heartbeat for the Responses startup frame. Comments keep the HTTP connection alive without creating a fake response/output item or polluting the Responses lifecycle. The route continues to forward real response.created and response.completed events unchanged.

Also add coverage for the observed nested response.failed + server_is_overloaded SSE shape so the HTTP Codex capacity fallback converts it to 503.

Verification

• DISABLE_SQLITE_AUTO_BACKUP=true node --import tsx/esm --import ./open-sse/utils/setupPolyfill.ts --test tests/unit/early-stream-keepalive.test.ts (17 passing)
• node --import tsx/esm --test tests/unit/early-stream-keepalive.test.ts tests/unit/codex-sse-capacity-fallback.test.ts (21 passing)
• git diff --check

[tufw95]

Follow-up commit bfb0901b adds bounded Codex account rotation for transient upstream 5xx overloads.

Evidence: NAS logs showed repeated Codex Our servers are currently overloaded responses normalized by Omni to HTTP 502; v3.8.49's direct Codex failover branch only rotated on 429, so no same-request CODEX_FAILOVER occurred. This patch rotates only codex requests on 429/502/503/504 (max existing 3 attempts), excludes failed connection IDs, records a per-model temporary scope cooldown, and preserves non-Codex and arbitrary 5xx behavior. The existing SSE peek still maps nested response.failed/server_is_overloaded 200 streams to 503.

Focused tests:

• node --test --import tsx tests/unit/codex-failover.test.ts tests/unit/codex-sse-capacity-fallback.test.ts — 15 passed
• git diff --check — passed

Not merged or deployed.

[tufw95]

Amended follow-up commit to 2269b20 (same patch; removed unused helper parameter). Tests unchanged: 15 focused Codex tests pass.