Session-recovery robustness (from PR #1)

- Suppress ConnectionError log noise from in-flight requests draining
  after a session close (poll_scheduler + keepalive log at DEBUG).
- Null KeepaliveTask.on_unreachable during a forced reconnect so the
  dying session's keepalive can't flip HA availability offline after
  the new session is already healthy. Ported into PR #5's
  _maybe_force_reconnect flow.

Drops the _session_stop / publish-health force-close mechanism from
the original PR — PR #5's last_success_ts + _maybe_force_reconnect
already covers the 'session dead, restart it' goal via a different
path, and running both means two paths force-closing the same session
on the same failure.

samsung_appliance/keepalive.py

@@ -67,6 +67,8 @@ class KeepaliveTask:
         try:
             self.session.ping()
             ok = True
+        except ConnectionError as e:
+            if self.log: self.log.debug("ping: %s", e)
         except Exception as e:
             if self.log: self.log.warning("ping: %s", e)
         # Real half-open detection: ping sends can succeed against a