upsert method

Implementation

@action
void upsert(ModelBaseMessage msg) {
  if (!_map.containsKey(msg.id)) {
    // For new messages, add to order list
    _order.add(msg.id);
  }
  _map[msg.id] = msg;
  _messageStatusMap[msg.id] = msg.status;
  // Always resort to maintain correct order
  // This handles both new insertions and updates
  _order.sort((a, b) {
    final seqA = _map[a]?.sequence ?? 0;
    final seqB = _map[b]?.sequence ?? 0;
    return seqB.compareTo(seqA); // Descending order (newer messages first)
  });
}