## Why
Queue-only child-agent updates that arrive around a final answer should not
restart sampling, but they must remain available to the next turn. Explicitly
injected response items still need to reopen the current turn.
## What changed
- Defer queue-only inter-agent mail after a final answer without treating it as
same-turn pending input.
- Re-enable current-turn mailbox delivery when the model requests a follow-up,
a stop hook continues the turn, or a response item is explicitly injected.
- Check for turn-triggering mailbox work after clearing the completed active
turn so pending work can start safely.
## Testing
Added coverage that queued child mail waits for the next user turn and that an
injected response item after a final answer triggers a follow-up request.
GitOrigin-RevId: aa81d00707943284b7ab069b60970e547908c895
jif
c28770a42f