Challenge 1: Implement getRoomMembers — Possible Solution
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function getRoomMembers(roomName) {
  const members = rooms.get(roomName);
  if (!members) return [];

  const userIds = [];
  for (const socket of members) {
    if (socket.userId) {
      userIds.push(socket.userId);
    }
  }
  return userIds;
}

WHY THIS WORKS
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- rooms.get(roomName) returns the Set of sockets currently in that room
  — the exact same Set that sendToRoom() iterates over for broadcasting,
  reused here for a read-only purpose instead.
- If the room doesn't exist at all (nobody has ever joined it, or
  everyone has left and it was never cleaned up), rooms.get() returns
  undefined — returning an empty array in that case avoids the caller
  needing to handle "undefined" as a special case for a room with zero
  members versus a room that's never existed.
- "if (socket.userId)" filters out any socket that doesn't have a
  userId assigned — this matters because not every connected socket is
  necessarily guaranteed to have completed whatever identifies it (e.g.
  a connection mid-authentication), so a defensive check here avoids
  surfacing an undefined entry in the returned list.
- Returning a plain array of userIds (rather than the Set of sockets
  themselves) is deliberate: the caller building a "who's here" UI list
  needs simple, serializable user identifiers, not live socket objects —
  exposing the raw sockets would leak internal connection state to
  wherever this function's result eventually gets used (e.g. sent to a
  client as JSON).