Recent messages in an item's chat

Read the recent messages in an item's chat: what the user said there, what Roma replied, and what any agent posted with post_chat_message. On a shared item (shared on its own or through its list) the chat is its Thread, the one conversation the owner and every member see, so you read what the members said too. Pass since to read only what arrived after a moment you already saw; a read that repeats on a schedule passes the createdAt of the last message it saw every time. Empty when the item has no chat yet.