Clarify session exit messages (#40670)

## What changed

- Report `Session archived: <thread-id>` after successfully archiving the current session.
- Show the session ID when no resume hint is available, including for non-fatal exits.
- Keep deleted-thread exits distinct from successful archive exits.

## Testing

- Add coverage for archive lifecycle handling and exit-message formatting.

GitOrigin-RevId: 1812374e4c74fa1b3f11feb323de18a9dbff745b