fix(stream): reduce keepalive interval and handle sink closed gracefully

[85339098-afk]

Fix two root causes for stream disconnect. See full description in commit.

[jason837]

Thanks for digging into #60/#65 🙏 Splitting the two changes:

Change 1 (keepalive 15s→5s + env override): 👍 happy to take this. Lowering the keepalive cadence is very plausibly the actual root cause for the disconnects under MiMo thinking mode. One nit: Number(x) || 5000 doesn't guard negatives — MIMO2CODEX_KEEPALIVE_MS=-1 becomes setInterval(fn, -1), which Node clamps to 1ms (keepalive storm). Could you wrap it, e.g. Math.max(1000, Number(...) || 5000)?

Change 2 (setImmediate yield): I don't think this does what the comment says, could you double-check it actually helps in isolation? Walking the call chain:

Upstream cancellation is already handled — req.on("close") → ac.abort() (server.ts:737-738) aborts the upstream fetch(..., { signal }), so the connection is torn down regardless of this yield.
The for await return already triggers the generator's .return() → finally { reader.releaseLock() } by spec; a prior event-loop tick doesn't change that.
This branch is a normal return (incomplete), not a throw, so it never reaches the catch in handleResponses, and sink.closed() is already true so no SSE error event can be emitted anyway.
So as far as I can tell the await new Promise(r => setImmediate(r)) is a no-op for cleanup. If you saw it change behavior, can you share the repro? Otherwise I'd suggest dropping it (or at least correcting the comment) so we don't leave a misleading note behind.

Last thing — per the repo's change-log rule, this needs entries in doc/tag-log.md + doc/tag-log.zh.md and a ReleaseHighlight in web/src/release-notes.tsx. Could you add those for the keepalive change? Then this is good to merge. Thanks again!