[BUG]循环引用bug

### 问题描述
*简要描述您碰到的问题。*
升级2.X api版本默认以及api不兼容，不符合JSON规范输出

### 环境信息
*请填写以下信息：*

- OS信息： [e.g.：CentOS 8.4.2105 4Core 3.10GHz 16 GB]
- JDK信息： [e.g.：Openjdk 1.8.0_312]
- 版本信息：[e.g.：Fastjson2 2.x.x]
之前是1.2.83版本

com.alibaba
fastjson
2.0.64

### 重现步骤
*如何操作可以重现该问题：*

1. 使用 `xxx.xxx` 方法
2. 输入 `...` 数据
3. 出现 `...` 错误
```java
import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.alibaba.fastjson.serializer.SerializerFeature;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
* FastJSON2 循环引用 / $ref 场景自测。
*

* IDEA：右键 Run {@link #main(String[])}。
* 关注控制台 JSON 里是否出现 {@code "$ref"}。
*/
public class Fastjson2RefScenariosTest {

public static void main(String[] args) {
System.out.println("fastjson version = " + JSON.VERSION);
System.out.println("--------------------------------------------------");

run("1. 共享 Integer", sharedInteger(), false);
run("2. 共享 String", sharedString(), false);
run("3. Map 两 key 同一 List", mapSharedList(), true);
run("4. Map 两 key 同一 Map 实例", mapSharedMap(), true);
run("5. LimitGroup POJO + 共享 List（插件 getLimitGroup）", pojoSharedList(), false);
run("6. LimitGroup 共享 Set", flashSharedSet(), false);
run("7. getAttr：多次 JSON.toJSON(dataMap)", getAttrStyleJsonToJson(), false);
run("8. getAttr：同一 Map 实例放两 key", sameMapTwoKeys(), true);
run("9. limit 用 Map 结构 + 共享 List", mapLimitSharedList(), true);
run("10. 完整 Req：attr 共享 Map + limit POJO", fullReq(), true);
run("11. 同一 JSONObject 放两 key", sharedJSONObject(), true);
run("12. 数组两元素同一 List", arraySameObject(), false);
run("13. 真循环 parent<->child", circular(), true);

System.out.println("--------------------------------------------------");
System.out.println("DisableCircularReferenceDetect 修复示例（场景 3）：");
String fixed = JSON.toJSONString(mapSharedList(), SerializerFeature.DisableCircularReferenceDetect);
System.out.println(fixed);
System.out.println("has $ref: " + fixed.contains("$ref"));
}

private static void run(String title, Object obj, boolean expectRef) {
String json = JSON.toJSONString(obj);
boolean hasRef = json.contains("$ref");
String mark = hasRef == expectRef ? "OK" : "UNEXPECTED";
System.out.println("\n=== " + title + " [" + mark + ", expect $ref=" + expectRef + "] ===");
System.out.println(json);
System.out.println("has $ref: " + hasRef);
}

private static Map sharedInteger() {
Integer v = Integer.valueOf(6);
Map m = new LinkedHashMap<>();
m.put("a", v);
m.put("b", v);
return m;
}

private static Map sharedString() {
String v = "CN";
Map m = new LinkedHashMap<>();
m.put("a", v);
m.put("b", v);
return m;
}

private static Map mapSharedList() {
List list = new ArrayList<>();
list.add("-1");
Map m = new LinkedHashMap<>();
m.put("area", list);
m.put("macs", list);
return m;
}

private static Map mapSharedMap() {
Map inner = new LinkedHashMap<>();
inner.put("data", new ArrayList());
Map m = new LinkedHashMap<>();
m.put("areaList", inner);
m.put("macs", inner);
return m;
}

private static Req pojoSharedList() {
List list = new ArrayList<>();
list.add("-1");
List rules = new ArrayList<>();
Rule r1 = new Rule();
r1.ruleName = "area";
r1.ruleType = 1;
r1.value = list;
Rule r2 = new Rule();
r2.ruleName = "macs";
r2.ruleType = 1;
r2.value = list;
rules.add(r1);
rules.add(r2);
LimitGroup g = new LimitGroup();
g.groupName = "DEFAULT";
g.rules = rules;
Req req = new Req();
req.operatorId = "PLUGIN";
req.limit = Collections.singletonList(g);
return req;
}

private static Req flashSharedSet() {
Set areas = new LinkedHashSet<>(Arrays.asList("CN", "IN"));
List groups = new ArrayList<>();
for (int i = 0; i < 2; i++) {
List rules = new ArrayList<>();
Rule area = new Rule();
area.ruleName = "publishArea";
area.ruleType = 1;
area.value = areas;
Rule other = new Rule();
other.ruleName = i == 0 ? "marketModel" : "versionPackageName";
other.ruleType = 6;
other.value = "CPH2173";
rules.add(area);
rules.add(other);
LimitGroup g = new LimitGroup();
g.groupName = "NUM_" + i;
g.rules = rules;
groups.add(g);
}
Req req = new Req();
req.operatorId = "grs";
req.limit = groups;
return req;
}

private static Map getAttrStyleJsonToJson() {
HashMap dataMap = new HashMap<>();
dataMap.put("data", new ArrayList());
HashMap resultMap = new HashMap<>();
resultMap.put("pluginCode", JSON.toJSON(dataMap));
dataMap.put("data", "v1");
resultMap.put("version", JSON.toJSON(dataMap));
return resultMap;
}

private static Map sameMapTwoKeys() {
Map dataMap = new HashMap<>();
dataMap.put("data", new ArrayList());
Map resultMap = new LinkedHashMap<>();
resultMap.put("areaList", dataMap);
resultMap.put("macs", dataMap);
return resultMap;
}

private static Map mapLimitSharedList() {
List list = new ArrayList<>();
list.add("-1");
Map areaRule = new LinkedHashMap<>();
areaRule.put("ruleName", "area");
areaRule.put("value", list);
Map macRule = new LinkedHashMap<>();
macRule.put("ruleName", "macs");
macRule.put("value", list);
Map group = new LinkedHashMap<>();
group.put("groupName", "DEFAULT");
group.put("rules", Arrays.asList(areaRule, macRule));
Map req = new LinkedHashMap<>();
req.put("limit", Collections.singletonList(group));
return req;
}

private static Req fullReq() {
Req req = pojoSharedList();
req.attr = sameMapTwoKeys();
return req;
}

private static Map sharedJSONObject() {
JSONObject jo = new JSONObject();
jo.put("data", new ArrayList());
Map m = new LinkedHashMap<>();
m.put("areaList", jo);
m.put("macs", jo);
return m;
}

private static Map arraySameObject() {
List list = new ArrayList<>();
list.add("CN");
Map m = new LinkedHashMap<>();
m.put("items", Arrays.asList(list, list));
return m;
}

private static Map circular() {
Map parent = new LinkedHashMap<>();
Map child = new LinkedHashMap<>();
parent.put("name", "p");
parent.put("child", child);
child.put("name", "c");
child.put("parent", parent);
return parent;
}

static class Rule {
public String ruleName;
public int ruleType;
public Object value;
}

static class LimitGroup {
public String groupName;
public List rules;
}

static class Req {
public String operatorId;
public Object limit;
public Map attr;
}
}

```

### 期待的正确结果
*对您期望发生的结果进行清晰简洁的描述。*
希望对于引用这类在java中那是百分之百存在的；但是fastjson却默认序列化有问题；导致传递给下游字段值是$.ref

### 相关日志输出
*请复制并粘贴任何相关的日志输出。*

#### 附加信息
*如果你还有其他需要提供的信息，可以在这里填写（可以提供截图、视频等）。*
非常感谢项目开源奉献，历史旧项目改不动只能升级，但希望项目开源版本前使用AI自测各种API兼容性；不兼容的能明确告知，输出明确差异点；不要开源后拿用户当测试；

Start with the supplied Fastjson2RefScenariosTest.main entry point and run its scenarios against Fastjson 2.0.64, comparing the observed output with the reported Fastjson 1.2.83 behavior. The issue needs a decided compatibility expectation for shared references versus true circular references, with the relevant differences and intended JSON output documented before implementation.

Written by the indexing model from the issue text.