fix(box): detect disconnect when handler.run() returns normally

The generic Handler.run() catches ConnectionClosedError and breaks out
of its loop (normal return) instead of raising, because it has no
disconnect_callback. The old code only triggered reconnection in the
except branch, so a clean WebSocket close was never detected.

Now treat handler.run() returning normally (after successful handshake)
as a disconnect event, triggering the reconnection callback.

src/langbot/pkg/box/connector.py

@@ -251,9 +251,12 @@ class BoxRuntimeConnector(ManagedRuntimeConnector):
                 if not connected.is_set():
                     connect_error.append(exc)
                     connected.set()
-                else:
-                    # Connection was established but then dropped — this is a
-                    # disconnect after successful handshake.
+                    return
+
+            # If we reach here, handler.run() returned normally (connection
+            # closed) or raised after the initial handshake succeeded.
+            # Either way, treat it as a disconnect.
+            if connected.is_set():
                 if self._uses_websocket():
                     self.ap.logger.error('Disconnected from Box runtime, trying to reconnect...')
                     if self.runtime_disconnect_callback is not None: