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).

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This section details on the original issue you should resolve

<issue_title>退群和加群的时候，变更成员的字段都是空字符串</issue_title>
<issue_description>这个问题没有解决，将切换至4.8.0和4.8.1.B这两个版本测试，都是空字符串，包含退群和加群的时候，这个变更成员的字段都是空字符串
log.debug("解密后的原始xml消息内容：{}", plainText)
<xml><ToUserName><![CDATA[c2e112dad808119117371bbcd6]]></ToUserName><FromUserName><![CDATA[sys]]></FromUserName><CreateTime>9811170016713</CreateTime><MsgType><![CDATA[event]]></MsgType><Event><![CDATA[change_external_chat]]></Event><ChatId><![CDATA[wrxUBwDQAAa44T11Ziaed811rhUr8-3Igmug]]></ChatId><ChangeType><![CDATA[update]]></ChangeType><UpdateDetail><![CDATA[add_member]]></UpdateDetail><JoinScene>3</JoinScene><MemChangeCnt>1</MemChangeCnt><MemChangeList><Item><![CDATA[wmxUBwDQAAO-Hn5_wFJz4wvo5TxLFibw]]></Item></MemChangeList><LastMemVer><![CDATA[5807afd2ab75771d5e8ac623f534ac0b]]></LastMemVer><CurMemVer><![CDATA[ea36e8b6062b803cda0ee45e9418d637]]></CurMemVer></xml>

Originally posted by @zhouHongGui in #3827 (comment)
</issue_description>

Comments on the Issue (you are @copilot in this section)

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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