[QUESTION] WriteClassName 两级以上的内嵌对象没有生成@type

### 请描述您的问题
*询问有关本项目的使用和其他方面的相关问题。*
```
public class Test {

public static void main(String[] args) {
Person p = new Person();
p.setContact(new Contact());
System.out.println(com.alibaba.fastjson2.JSON.toJSONString(new RpcResult<>(p),
JSONWriter.Feature.WriteClassName,
// JSONWriter.Feature.WriteEnumUsingToString,
JSONWriter.Feature.WriteMapNullValue));
}
}
```

生成的结果：
```
{"@type":"com.xxx.RpcResult","data":{"@type":"com.xxx.model.Person","active":null,"activeTime":null,"birthday":null,"companyName":null,"contact":{"privateContact":null,"publicContact":null},"createTime":null,"department":null,"eid":null,"email":null,"fullPinyin":null,"gender":null,"hide":false,"id":null,"isAdmin":0,"isHidePhone":0,"jobNo":null,"jobTitle":null,"name":null,"oId":null,"openId":null,"orgId":null,"orgInfoId":null,"phone":null,"photoUrl":null,"registerDate":null,"status":0,"tid":null,"uid":null,"updateTime":null,"userName":null,"userType":0,"wbNetworkId":null,"wbUserId":null,"weights":0},"error":null,"errorCode":0,"success":true}
```
期望的结果：
```
{"@type":"com.xxx.RpcResult","data":{"@type":"com.xxx.model.Person","active":null,"activeTime":null,"birthday":null,"companyName":null,"contact":{"@type":"com.xxx.model.Contact","privateContact":null,"publicContact":null},"createTime":null,"department":null,"eid":null,"email":null,"fullPinyin":null,"gender":null,"hide":false,"id":null,"isAdmin":0,"isHidePhone":0,"jobNo":null,"jobTitle":null,"name":null,"oId":null,"openId":null,"orgId":null,"orgInfoId":null,"phone":null,"photoUrl":null,"registerDate":null,"status":0,"tid":null,"uid":null,"updateTime":null,"userName":null,"userType":0,"wbNetworkId":null,"wbUserId":null,"weights":0},"error":null,"errorCode":0,"success":true}
```

Reproduce the nested-object case from the issue using JSON.toJSONString with JSONWriter.Feature.WriteClassName and WriteMapNullValue. Trace why the Contact object inside Person is missing @type, and consider the work complete when the output includes com.xxx.model.Contact as shown in the expected JSON.

Written by the indexing model from the issue text.