fix: preserve background notifications after end_turn

[Gujiassh]

Summary

• keep the s08 and s_full loops alive until queued background task notifications are actually delivered after an end_turn
• preserve the existing no-assistant-ack message ordering from current main while still injecting the background results as a user block
• add focused regression tests for both entrypoints

End-user benefit

When a long-running background command finishes right after the model returns end_turn, the current examples can exit before the result is ever surfaced back into the conversation. This change makes the background result reliably show up in the transcript instead of silently disappearing.

Testing

• python -m pytest tests/test_background_notifications.py -q
• python -m py_compile agents/s08_background_tasks.py agents/s_full.py tests/test_background_notifications.py

[vercel]

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[Gujiassh]

Follow-up note: this branch was rebuilt from current main around the background-notification wait logic, and the focused regression checks pass locally (python -m pytest tests/test_background_notifications.py -q, python -m py_compile agents/s08_background_tasks.py agents/s_full.py tests/test_background_notifications.py). The visible GitHub blockers remain the two Vercel team-authorization statuses; code-side failures are not currently showing on this branch.

[Gujiassh]

Follow-up note: I rebuilt this branch again from the latest available main snapshot, preserved the no-assistant-ack contract, and reran the focused background notification regression checks locally (python -m pytest tests/test_background_notifications.py -q, python -m py_compile agents/s08_background_tasks.py agents/s_full.py tests/test_background_notifications.py). The visible GitHub blockers remain the two Vercel team-authorization statuses.

[Gujiassh]

Status update: after the latest refresh, this branch still has no visible code-side CI failures. The only remaining blockers are the two Vercel team-authorization statuses on the project account side.

[Gujiassh]

Follow-up note: I rebuilt this branch once more directly on top of the latest current main, so it now carries only the background-notification wait fix and its focused regression tests. The refreshed head is e62d5ae7d, and the only visible blockers should remain the Vercel team-authorization statuses.