Load message history in reverse

Use this method when you already have a message anchor and need to load the messages that follow it:

final page = await OpenIM.iMManager.messageManager
    .getAdvancedHistoryMessageListReverse(
  conversationID: conversationID,
  startMsg: anchorMessage,
  count: 30,
  viewType: GetHistoryViewType.history,
);

Do not mix both loading directions at the same scrolling boundary. The Future returns a message snapshot in the requested direction and does not trigger the new-message listener. When merging, keep the result within its conversationID and require a non-empty clientMsgID.