Challenge 1: Add a "Leave Room" Message Type — Possible Solution
====================================================================

socket.on("message", (raw) => {
  let data;
  try { data = JSON.parse(raw.toString()); } catch { return; }
  if (!validateChatMessage(data)) return;

  if (data.type === "join") {
    joinRoom(socket, data.roomName);
  }

  if (data.type === "leave") {
    leaveRoom(socket, data.roomName); // explicit leave, not waiting for close
  }

  if (data.type === "chat") {
    sendToRoom(data.roomName, JSON.stringify({
      type: "chat", userId: socket.userId, text: data.text
    }));
  }
});

// validateChatMessage would also need to accept "leave" as a valid type
// alongside "join" and "chat", still requiring a valid roomName field.

WHY THIS WORKS
--------------
- The new "leave" branch calls the EXACT SAME leaveRoom(socket, roomName)
  function from Chapter 6 that the "close" event handler already calls
  — there's no separate "leave" implementation to write, just a new
  message type that triggers the existing function explicitly, while
  the user is still connected.
- This matters because a user might want to leave one specific room
  while staying connected overall (e.g. switching from the "lobby" room
  to a "support" room) — relying only on the close event's cleanup would
  mean the only way to leave a room is to disconnect entirely, which
  isn't how a real multi-room chat feature typically behaves.
- leaveRoom already handles removing the socket from BOTH the room's
  Set and the socket's own socket.rooms tracking Set (Chapter 6) — so
  after an explicit "leave" message, the close handler's cleanup loop
  (which iterates socket.rooms) correctly no longer includes that room,
  since it was already removed here.
- Both the new "leave" branch and the existing "join"/"chat" branches
  still run through the same validateChatMessage(data) check before any
  of this logic executes — the new message type doesn't bypass the
  shape-validation step this course's security chapter established for
  every incoming message, regardless of type.