agent/engine: keep cron/hook client alive for live background tasks

[polyglotAI-bot]

Summary

One-shot runs (run_cron / run_hook) tore down the SDK client the instant the agent yielded its turn — even when a Bash(run_in_background) task was still running. Discarding kills the CLI subprocess and the idle-stream watcher that delivers the background task's completion turn, so the task is orphaned and the run never resumes to finish its work (push, update state, notify). This stranded isolated cron workers: a run kicked off a build in the background, ended its turn to "await the completion notification", and was never woken.

Route the three one-shot teardowns through a new _teardown_oneshot_client() that skips the discard while _has_live_background_tasks() is true — the same predicate run_idle_client_sweep already uses to keep such clients alive (#125). The watcher then delivers the completion turn (the agent resumes and finishes) and the idle sweep reaps the client once the task settles — exactly the lifecycle a web/interactive session already has.

Why this is minimal

The capability already worked for web/persistent sessions: the idle-stream watcher delivers a background task's completion as an autonomous turn, and the idle sweep already refuses to discard a client with a live background task. The only gap was the one-shot teardown discarding unconditionally. This makes the cron/hook teardown consistent with that existing guard.

run_persistent_cron is intentionally excluded

Its session_id is stable and reused across runs, so parking the client on a live background task would let the next scheduled run reuse the same client/conversation while the prior task is still in flight. Keep-alive is therefore restricted to the unique-per-run isolated paths (run_cron, run_hook); persistent crons discard as before (keepalive_if_bg=False).

Out of scope (pre-existing)

A background task that never settles parks the subprocess indefinitely — but that is already true for web/interactive sessions today (the sweep skips live-bg sessions unconditionally). Bounding it is a separate, general idle-sweep change for all session types, not cron-specific.

Test plan

• run_cron / run_hook keep the client alive when a background task is live; run_persistent_cron discards even with a live task.
• End-to-end: driving the real _idle_stream_watcher against a parked client, a background-completion sequence is delivered as exactly one autonomous resume turn, and the task flips out of "live" so the sweep can reap it.
• No regression — affected engine/cron/session suites pass (247). Full suite green except 7 pre-existing, unrelated test_codex_* failures (confirmed failing on main without this change).

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