seedRoomMeta method
Test/demo helper: seed chat-list metadata (unread badge, pinned,
muted) that the real backend computes/stores but the mock ChatRoom
model doesn't carry. Null args leave the current value untouched.
Implementation
void seedRoomMeta(String roomId, {int? unread, bool? pinned, bool? muted}) {
if (unread != null) _unread[roomId] = unread;
if (pinned != null) _pinned[roomId] = pinned;
if (muted != null) _muted[roomId] = muted;
}