fix(responses): refuse oversized input and reframe routed compaction to SSE (#1128)

[HoshimiRox1]

Summary

Two fixes on the Codex desktop context/compaction failure chain (reported upstream in #1128):

• refuse responses input beyond the advertised context window — a chained-turn replay can balloon a request far past the model's context window (observed: a ~400k-token conversation replayed as a 1.6M-token input). Forwarding it on Windows ballooned bun RSS and native-crashed the whole proxy service (upstream Bun memory bug, 램 누수가 있어요 #314), taking every active thread down until restart. The proxy now rejects such requests with a clean 413 before any upstream I/O; the client compacts well before the window, so the guard only fires on abnormal duplication.
• reframe routed compaction JSON to SSE for streaming clients (Fixes [Bug]: Remote compaction always fails on deepseek-v4-flash - the #1026 bounded-JSON policy is not applied on the compact path #1128) — the [Bug] DeepSeek V4 Flash Responses route stalls after tool calls #875/fix(deepseek): transport-neutral bounded-JSON policy with terminal SSE synthesis (#875) #1026 bounded-JSON policy forces a bounded-JSON upstream for deepseek-v4-flash even when the client asked for SSE. The passthrough branch already reframes that JSON into the canonical terminal SSE sequence, but routed-compaction turns returned application/json, so Codex's SSE parser hit EOF and remote compaction failed deterministically with "stream closed before response.completed", retried 6x, and left the thread unusable. Both routed return points (runTurn and parseStream) now apply the same reframe.

Verification

• bun run typecheck — clean.
• bun test tests/responses-input-guard.test.ts — 4 pass / 0 fail (guard above/below the window; compaction reframe with and without stream: true).
• Live verification against a patched instance: a deepseek-v4-flash compaction turn now returns response.created -> output_item.done (compaction) -> response.completed -> [DONE] instead of bare JSON; an oversized input is rejected with 413 without reaching upstream.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed. (bug fix; no user-facing docs change — release note is maintainer-managed)
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults. (no credentials touched; rejection path only)

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[github-actions]

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• review readiness checklist open (0/4 boxes ticked).

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• Tick all four boxes in the PR description once you're done (currently 0/4).

Review readiness checklist

• ⬜ All CI tests are green on my local testing.
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• ⬜ I resolved all correct Codex and CodeRabbit findings.
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[HoshimiRox1]

Closing for now — will re-open after reviewing the process. Thanks!