## What changed
- Handle app-server `turn/start` rejections for pending user turns as turn
errors instead of exiting the TUI.
- Finalize the pending turn, display the failure in the transcript, and resume
queued input handling.
- Add coverage that submits a turn to a missing thread and verifies that the
error is shown while the app continues running.
GitOrigin-RevId: 6d291ba62b9e50d49dd4e3d7c429e98c1f754680