[BUG]fastjson2 2.0.64 使用getBigDecimal存在精度丢失问题

### 问题描述
fastjson2 2.0.64 使用getBigDecimal存在精度丢失问题

### 环境信息
- OS信息： Mac Tahoe 26.1
- JDK信息：OpenJDK 21.0.7
- 版本信息：2.0.64

### 重现步骤
public static void main(String[] args) {
JSONObject value = new JSONObject().fluentPut("value", 0.00054797);
System.out.println(value.getBigDecimal("value"));
}
输出：0.0005479699999999999，与预期0.00054797不符。

Start by reproducing the example with JSONObject.fluentPut and getBigDecimal on fastjson2 2.0.64, then trace the implementation of those methods to find where the decimal value loses precision. Add a regression test covering 0.00054797 and verify that the returned value matches the expected decimal representation.

Written by the indexing model from the issue text.