GeezerVison

WxCpXmlMessage.getMemChangeList() returns "" for add/remove member events because the WeChat API switched <MemChangeList> from a flat CDATA string to a nested <Item> element list, which the old XStreamCDataConverter cannot handle.

Old format (pre-4.8.0):

<MemChangeList><![CDATA[id1,id2]]></MemChangeList>

New format (4.8.0+):

<MemChangeList>
  <Item><![CDATA[id1]]></Item>
  <Item><![CDATA[id2]]></Item>
</MemChangeList>

Changes

• XStreamCDataListConverter (new) — Converter implementation that detects child elements at unmarshal time: iterates <Item> nodes and joins values with ,; falls back to direct CDATA text for the old format. Returns null (not "") when the element is absent.
• WxCpXmlMessage — switches memChangeList field annotation from XStreamCDataConverter to XStreamCDataListConverter.
• WxCpXmlMessageTest — adds testExternalChatChangeEvent covering: old flat CDATA (single/multi), new <Item> format (single/multi), and absent MemChangeList (null assertion).

Custom agent used: 全部用中文
需要用中文，包括PR标题和分析总结过程

• Fixes 退群和加群的时候，变更成员的字段都是空字符串 #3904

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