Fix the replay of a chat screen's sockets

Your React Native chat screen shows each message two, three, four times as users navigate in and out, and memory keeps climbing. Its hook opened a socket in an effect but returned no cleanup, so old listeners stayed alive. A cleanup is the function an effect returns, which React runs before the effect runs again and on unmount. To check the fix, the team wants a replay of the screen with the cleanup in place.

An assistant wrote replay_chat_messages(events). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• events: the event log, as strings, in order. It can be empty.
• Return the messages the screen holds after the last event, in the order they arrived.

Rules the tests check:

• Events and room names are case sensitive. At most one socket is open at any time.
• "mount:<room>": the screen mounts. Any open socket closes first. The message list becomes empty, and the screen listens to <room>. An empty room name means ignore the event.
• "room:<room>": the room changes while the screen is mounted. The old socket closes and a socket for <room> opens. The message list is kept. Ignore the event when the screen is not mounted or the room name is empty.
• "unmount": the socket closes, the screen is no longer mounted, and the message list becomes empty.
• "emit:<room>|<text>": the server sends <text> to <room>. The room is everything before the first "|" and the text is everything after it, so the text can be empty or contain "|". With no "|", ignore the event.
• An emit adds its text to the list exactly once when the screen is mounted and listening to that room. Otherwise it adds nothing.
• Ignore any other event.