[Bug]: Too mush token cost for [Demo PG - GAAP use case ]

### Summary

5,014,326 token cost (about $2) in https://bailian.console.aliyun.com for task
```
datapaw run --datasource-id postgresql-demo-gaap "分析 2026 年 3 月 product X 有效用户的平均 GAAP 值，展示时间趋势，并结合相关 KG 事件解释异常峰值"
```

### Minimal reproduction

minimal demo as README

### Environment

minimal demo as README

### Sanitized logs

```shell

```

### Checklist

- [x] I searched existing issues and discussions.
- [x] I removed secrets, private URLs, and business data.
- [x] This is not a security vulnerability.