[Fix] API shutdown can hang before Fast answers start

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Mark Fast shutdown closeout delivery as pending only when inference enters its cancellation-aware retry path.
• Release locks that are still in provider history, attachment, or session setup without waiting for a closeout resolver that cannot run.
• Preserve the existing graceful wait for answers that are active, and preserve rejection of lock acquisitions racing with process shutdown.

Why this change was made

This is a standalone follow-up to the review finding on PR #1950, based on the latest develop branch. A SIGTERM arriving while pre-answer setup was stalled could leave abortForShutdown waiting indefinitely and prevent the API's graceful shutdown from completing.

Impact

API shutdown no longer depends on pre-answer or answer-service setup reaching cancellation-aware inference. Active inference still receives the existing opportunity to post and persist its interruption closeout before its Redis turn lock is released. PR #1950 is not modified by this change.

Validation

• pnpm exec dotenvx run -f .env.test -- pnpm --filter @roomote/cloud-agents exec vitest run src/server/fast-agent/__tests__/fast-agent-turn-lock.test.ts src/server/fast-agent/__tests__/fast-agent-service.test.ts (107 tests passed)
• pnpm --filter @roomote/cloud-agents check-types
• pnpm --filter @roomote/cloud-agents format:check
• pnpm exec oxlint --deny-warnings packages/cloud-agents/src/server/fast-agent/fast-agent-turn-lock.ts packages/cloud-agents/src/server/fast-agent/fast-agent-service.ts packages/cloud-agents/src/server/fast-agent/__tests__/fast-agent-turn-lock.test.ts packages/cloud-agents/src/server/fast-agent/__tests__/fast-agent-service.test.ts
• Pre-push checks: oxlint, residual lint, check-types:fast, and knip

[roomote-community]

No code issues found. See task

• packages/cloud-agents/src/server/fast-agent/fast-agent-service.ts:872 Shutdown can still wait on uninterruptible answer-service setup.

_{Reviewed 8540d49}

[roomote-roomote]

Fixed the outstanding shutdown setup finding in 8540d49: closeout waiting now begins only inside cancellation-aware inference, and focused coverage exercises SIGTERM while session setup is stalled. See task.