[Fix] Deploys interrupt in-flight Fast turns instead of letting them finish

[daniel-lxs]

Problem

On SIGTERM the API aborts every active Fast turn as its first act, before anything else happens. Every deploy therefore kills whatever users have in flight and posts a restart closeout asking them to resend. Attribution work in #2004 identified this as the largest producer of interrupted-turn messages: the frequency is roughly deploys times concurrently active turns, and most Fast turns would have finished within seconds if given the chance.

Change

Shutdown now drains before it aborts:

1. beginFastAgentTurnDrain closes admissions immediately (new lock acquisitions are refused) without touching active turns.
2. server.close() stops accepting connections in parallel.
3. waitForActiveFastAgentTurnsToSettle gives in-flight turns a bounded window to finish on their own. It resolves the moment the last turn settles, so idle shutdowns stay instant, and the hard deadline means a wedged turn cannot hang shutdown (preserving the [Fix] API shutdown can hang before Fast answers start #1958 guarantee).
4. Only the stragglers still active at the deadline are aborted through the existing shutdown path, with the same closeout semantics as before, plus a log line stating how many turns were aborted after the drain.

The window is R_API_SHUTDOWN_DRAIN_MS (default 20s, sized to leave room for the straggler abort, closeout delivery, and Sentry flush inside a typical 30s SIGTERM-to-SIGKILL grace). Setting it to 0 restores the previous abort-immediately behavior as a kill switch. Documented in .env.production.example.

Validation

• New drain suite in @roomote/cloud-agents (fresh module instance per test): admissions refused during drain without aborting active turns, settle-wait resolves on last release, stragglers reported at the deadline, and stragglers abort with the reason the drain began with. Existing turn-lock suite unchanged and passing (11 tests total across both files).
• @roomote/api graceful-shutdown suite rewritten for the drain ordering: drain precedes abort, quiet path logs nothing, close-failure exit code and double-signal force-exit behavior preserved, env parsing covered (7 tests).
• pnpm lint:fast, pnpm check-types:fast, pnpm knip all pass.

Context

PR 2 of the interruption work started in #2004. Next: renewing the Fast responding lease mid-turn so the scheduled reconciler stops stamping turns that are still running.

[roomote-community]

No code issues found. See task

_{Reviewed 0d6ce08}