onItemAdded method
Called when a conversation is added to the list.
Implementation
@override
void onItemAdded(Conversation item, List<Conversation> updatedList) {
// OPTIMIZATION: Incremental map update - item added at end
final newIndex = updatedList.length - 1;
_addToIndexMap(item.conversationId, newIndex);
if (!isClosed) {
add(_ListItemAdded(updatedList));
}
}