Previously a failed connect() / start_notify() left self._client pointing
at a broken BleakClient. On the next poll is_connected returned False but
the stale object caused confusing errors. Now the client is only assigned
after both steps succeed; on any exception we try a clean disconnect and
reset to None so the next attempt starts completely fresh.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Jannis
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