docs: def subscribe(self, frame):
          """
          Handle the SUBSCRIBE command: Adds this connection to destination.
          """
          ack = frame.headers.get('ack')
          reliable = ack and ack.lower() == 'client'
  
          self.engine.connection.reliable_subscriber = reliable
  
          dest = frame.headers.get('destination')
          if not dest:
              raise ProtocolError('Missing destination for SUBSCRIBE command.')
  
          if dest.startswith('/queue/'):
              self.engine.queue_manager.subscribe(self.engine.connection, dest)
          else:
              self.engine.topic_manager.subscribe(self.engine.connection, dest)