Fix websocket "Cannot write to closing transport" error (#373)

Fixes #156

Prevent race condition where heartbeat ping attempts to send data
on a closing websocket connection:

- Add closed state check in _ping() before sending
- Wrap ping send in try/except to handle connection errors gracefully
- Cancel heartbeat callback when websocket closes or stops

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: sebr

custom_components/bhyve/pybhyve/websocket.py

@@ -67,9 +67,14 @@ def _send_heartbeat(self) -> None:
             self._loop.create_task(self._ping())
 
     async def _ping(self) -> None:
-        if self._ws is not None:
-            await self._ws.send_str(json.dumps({"event": "ping"}))
-            self._reset_heartbeat()
+        if self._ws is not None and not self._ws.closed:
+            try:
+                await self._ws.send_str(json.dumps({"event": "ping"}))
+                self._reset_heartbeat()
+            except ConnectionResetError:
+                _LOGGER.debug("Connection reset while sending ping")
+            except Exception as err:  # noqa: BLE001
+                _LOGGER.debug("Error sending ping: %s", err)
 
     @property
     def state(self) -> str:
@@ -146,6 +151,7 @@ async def running(self) -> None:  # noqa: PLR0912, PLR0915
 
                     if self._ws.closed:
                         _LOGGER.info("Websocket closed? %s", self._ws.closed)
+                        self._cancel_heartbeat()
                         self.state = STATE_STOPPED
 
                     if self._ws.exception():
@@ -173,6 +179,7 @@ async def stop(self) -> None:
         """Close websocket connection."""
         _LOGGER.info("Closing websocket connection; state: %s --> STOPPED", self.state)
         self.state = STATE_STOPPED
+        self._cancel_heartbeat()
         if self._ws is not None:
             await self._ws.close()