## Why
Completion receipts for identical user messages could acknowledge the wrong
pending steer. When the receipt arrived while another thread was displayed,
the stale pending state could later be restored and resubmitted.
## What changed
- Assign each TUI submission a client message ID and pass it through turn start
and steer requests.
- Preserve pending steer IDs across thread input snapshots and match completion
receipts by ID, while retaining content-based matching for older app servers
that do not echo IDs.
- Acknowledge matching receipts for inactive threads before their notifications
can be discarded, including threads stored in the agents overview.
## Testing
Added coverage for identical steer receipts and for switching within and
between tasks before interrupting an active turn.
GitOrigin-RevId: 28bd80a735ff86cd7bf836f24b448e7499f0a902
8b8ee28a9b