fix: disconnect() always clears _connected even if socket close fails

[cconstab]

In at_client/connections/atconnection.py:

def disconnect(self):
    self._secure_root_socket.close()   # raises on an already-broken socket
    self._connected = False            # skipped when close() raises

When the socket is already broken (e.g. Bad file descriptor), close() itself
raises, so _connected is never reset. The monitor restart path is guarded by
if not self._connected: self._connect(), so with _connected stuck at True it
never rebuilds the socket and keeps reading the dead descriptor — the client silently
stops receiving notifications until the whole AtClient is recreated.

Fix: wrap close() and always clear _connected in a finally.

Tests: test/disconnect_test.py — network-free; asserts _connected becomes
False both on a clean close and when close() raises OSError(Bad file descriptor).

This is the primary fix for the "monitor never recovers after a drop" behaviour;
see the related issue for the contributing factors (process-global monitor locks and a
non-daemon heartbeat thread that never exits).