[BUG] BigDecimal with @JsonFormat(shape = STRING) produces invalid JSON since 2.0.63

### 问题描述
BigDecimal 字段使用 Jackson 注解：

```java
@JsonFormat(shape = JsonFormat.Shape.STRING)
private BigDecimal amount
```
fastjson2 2.0.61 及 2.0.63 会生成非法 JSON：
```
{"amount":string200}
```
预期结果应为：
```
{"amount":"200"}
```
### 最小复现代码

### 环境信息
*请填写以下信息：*

OS信息： Windows 11 10.0 8Core amd64 64bit
JDK信息： Oracle Corporation 21.0.11
版本信息：2.0.63 和 1.8都测试了

### 重现步骤

```java
package org.example;

import com.alibaba.fastjson2.JSON;
import com.alibaba.fastjson2.JSONObject;
import com.alibaba.fastjson2.JSONWriter;
import com.alibaba.fastjson2.TypeReference;
import com.alibaba.fastjson2.annotation.JSONField;
import com.fasterxml.jackson.annotation.JsonFormat;

import java.math.BigDecimal;
import java.util.Date;

public class TestJson {
private String goodsUnit;
@JsonFormat(shape = JsonFormat.Shape.STRING)
private BigDecimal quantity;

@JsonFormat(pattern = "yyyy-MM-dd")
private Date invoiceDate;

public String getGoodsUnit() {
return goodsUnit;
}

public void setGoodsUnit(String goodsUnit) {
this.goodsUnit = goodsUnit;
}

public BigDecimal getQuantity() {
return quantity;
}

public void setQuantity(BigDecimal quantity) {
this.quantity = quantity;
}

public Date getInvoiceDate() {
return invoiceDate;
}

public void setInvoiceDate(Date invoiceDate) {
this.invoiceDate = invoiceDate;
}

public static void main(String[] args) throws Exception {
System.out.println(com.alibaba.fastjson.JSON.VERSION);
System.out.println(com.alibaba.fastjson2.JSON.VERSION);
String req = "{\"goodsUnit\":\"adfa\",\"quantity\":\"6.56000000001\",\"invoiceDate\":\"2026-07-30 10:57:08\"}";
TestJson testJson1 = com.alibaba.fastjson.JSONObject.parseObject(req,
TestJson.class);
System.out.println(com.alibaba.fastjson.JSON.toJSONString(testJson1));
TestJson testJson2 = com.alibaba.fastjson2.JSONObject.parseObject(req,
TestJson.class);
System.out.println(com.alibaba.fastjson2.JSON.toJSONString(testJson2));
}

}

```
1. 执行程序
```
2.0.61
2.0.61
{"goodsUnit":"adfa","invoiceDate":"2026-07-30","quantity":string7}
{"goodsUnit":"adfa","invoiceDate":"2026-07-30","quantity":string7}
```

### 期待的正确结果
期待返回
{"goodsUnit":"adfa","invoiceDate":"2026-07-30","quantity":“6.56000000001”}
@JsonFormat只为了在controller响应序列化使用，跟你内部使用无关

### 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
```

com.alibaba
fastjson
2.0.63


com.alibaba.fastjson2
fastjson2
2.0.63


com.fasterxml.jackson.core
jackson-databind
2.13.5

```

Start with the TestJson.main reproduction shown in the issue and compare fastjson2 serialization of the BigDecimal field annotated with @JsonFormat(shape = STRING). Verify that the output contains a quoted decimal value rather than string7 or string200, and add a regression test covering this case.

Written by the indexing model from the issue text.