## Why
Multi-agent exec sessions can receive child `turn/completed` notifications on
the same event stream as the primary turn. Backfilling those unrelated
completions issues unnecessary `thread/read` requests.
## What changed
Run completion-item backfill only after a notification has been matched to the
primary exec thread and turn.
## Testing
Add an integration test that delivers a child completion before the primary
completion and verifies that only the primary completion triggers backfill.
GitOrigin-RevId: 46a41236ee6a39e4b341570ff4826a71fa02bb32
