fix: don't block StatusMessageWatcher exit with 6s sleep on exception (#753)

## Summary

`StatusMessageWatcher` sleeps 6 seconds on exit to let the final
status/status-message propagate. Two issues with the current placement:

- **API asymmetry**: the sync `stop()` does the sleep itself, while the
async twin does it in `__aexit__`. So manual sync `stop()` paid the 6s
even though manual `await stop()` did not.
- **Exceptional exits were delayed**: both `__exit__` and `__aexit__`
ignored the `exc_type` they receive, so a `Ctrl+C` or any exception
bubbling out of `with`/`async with` was held up 6 seconds.

This change:

- Moves `time.sleep(self._final_sleep_time_s)` out of sync `stop()` into
`__exit__` (matches async shape).
- Gates the sleep in both `__exit__` and `__aexit__` on `exc_type is
None` so exceptional exits propagate immediately.

Normal-exit behavior (the final-message capture window) is preserved.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>