Update the assigned attendant (user_id) and/or the status of a chat. Assigning an attendant forces the chat status to OPEN, unless an explicit status is sent in the same request (the explicit status wins). Sending user_id: null unassigns the attendant without changing the status. Setting status to CLOSED records closed_at. At least one of user_id or status is required. Only chats belonging to the authenticated owner can be updated.