[BUG] parse `"$ref"` then get() triggers ClassCastException

### 问题描述
https://github.com/alibaba/fastjson2/blob/334d25696654bb4e0593db33c4ec47afaa217cd9/core/src/test/java/com/alibaba/fastjson2/v1issues/issue_3900/Issue3922.java#L13
fastjson兼容解析`"$ref"`的json对象后使用get出现ClassCastException。

### 环境信息

- OS信息： [MacOS 12.7.4 M1 Pro 16 GB]
- JDK信息： [Openjdk 17.0.6]
- 版本信息：[Fastjson 2.0.51]

### 重现步骤

```java

import com.alibaba.fastjson.JSONObject;
import org.json.JSONException;
import org.junit.jupiter.api.Test;
import org.skyscreamer.jsonassert.JSONAssert;

public class Issue3922 {
@Test
public void test_for_issue3922() throws JSONException {
String jsonString = "{\"this0\":{\"$ref\":\"1\"}}";
JSONObject jsonObject = JSONObject.parseObject(jsonString, JSONObject.class);
JSONObject innerObject = (JSONObject) jsonObject.get("this0");
System.out.println(innerObject.toString());
JSONAssert.assertEquals("{\"$ref\":\"1\"}", innerObject.toString(), true);
}
}
```

### 相关日志输出
java.lang.ClassCastException: class com.alibaba.fastjson2.JSONObject cannot be cast to class com.alibaba.fastjson.JSONObject (com.alibaba.fastjson2.JSONObject and com.alibaba.fastjson.JSONObject are in unnamed module of loader 'app')

Start with the reproducer in core/src/test/java/com/alibaba/fastjson2/v1issues/issue_3900/Issue3922.java, especially parseObject and the subsequent get("this0") cast. Run test_for_issue3922 to confirm the ClassCastException. Done means the compatibility path handles the "$ref" object without the reported cast failure and the existing JSONAssert check passes.

Written by the indexing model from the issue text.