feat(box): add heartbeat and reconnection for Box runtime connector

Add 20-second heartbeat ping loop to detect silent Box runtime
disconnections. On disconnect, set available=false and attempt
reconnection after 3 seconds via the disconnect callback chain.

- BoxRuntimeConnector: heartbeat loop, disconnect callback parameter,
  disconnect detection in connection callback and WS failure handler
- BoxService: wire disconnect callback to toggle available state and
  re-initialize the connector on reconnection

src/langbot/pkg/box/service.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 import collections
 import datetime as _dt
 import enum
@@ -52,7 +53,7 @@ class BoxService:
         self.ap = ap
         self._runtime_connector: BoxRuntimeConnector | None = None
         if client is None:
-            self._runtime_connector = BoxRuntimeConnector(ap)
+            self._runtime_connector = BoxRuntimeConnector(ap, runtime_disconnect_callback=self._on_runtime_disconnect)
             client = self._runtime_connector.client
         self.client = client
         self.output_limit_chars = output_limit_chars
@@ -82,6 +83,18 @@ class BoxService:
             self.ap.logger.warning(f'LangBot Box runtime unavailable, sandbox features disabled: {exc}')
             self._available = False
 
+    async def _on_runtime_disconnect(self, connector: BoxRuntimeConnector) -> None:
+        """Called by the connector when the Box runtime connection drops."""
+        self._available = False
+        self.ap.logger.warning('Box runtime disconnected, sandbox features temporarily disabled. Reconnecting in 3s...')
+        await asyncio.sleep(3)
+        try:
+            await connector.initialize()
+            self._available = True
+            self.ap.logger.info('Box runtime reconnected, sandbox features restored.')
+        except Exception as exc:
+            self.ap.logger.warning(f'Box runtime reconnection failed: {exc}. Will retry on next heartbeat disconnect.')
+
     @property
     def available(self) -> bool:
         return self._available