fix(box): handle RPC failure in get_status/get_sessions gracefully

When the Box runtime disconnects, there is a race between the heartbeat
flipping _available=false and the frontend polling get_status(). If the
poll arrives first, client.get_status() throws a ConnectionClosedError
which propagated as a 500, causing the frontend to show a grey dot
(null status) instead of a red dot with error details.

Now get_status() catches RPC errors and returns available=false with
the exception message as connector_error. get_sessions() returns an
empty list when unavailable or on RPC failure.

src/langbot/pkg/box/service.py

@@ -219,7 +219,12 @@ class BoxService:
             self._shutdown_task = loop.create_task(self.shutdown())
 
     async def get_sessions(self) -> list[dict]:
-        return await self.client.get_sessions()
+        if not self._available:
+            return []
+        try:
+            return await self.client.get_sessions()
+        except Exception:
+            return []
 
     def build_spec(self, spec_payload: dict, skip_host_mount_validation: bool = False) -> BoxSpec:
         spec_payload = dict(spec_payload)
@@ -581,7 +586,17 @@ class BoxService:
                 'recent_error_count': len(self._recent_errors),
                 'connector_error': self._connector_error,
             }
-        runtime_status = await self.client.get_status()
+        try:
+            runtime_status = await self.client.get_status()
+        except Exception as exc:
+            # RPC failed — the runtime likely just disconnected and the
+            # heartbeat hasn't flipped _available yet.
+            return {
+                'available': False,
+                'profile': self.profile.name,
+                'recent_error_count': len(self._recent_errors),
+                'connector_error': str(exc),
+            }
         return {
             **runtime_status,
             'available': True,

tests/unit_tests/box/test_box_service.py

@@ -182,6 +182,7 @@ async def test_box_service_get_sessions_delegates_to_client():
     client.get_sessions = AsyncMock(return_value=[{'session_id': 'test-session'}])
 
     service = BoxService(make_app(Mock()), client=client)
+    service._available = True
 
     sessions = await service.get_sessions()