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.

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()