[Bug] `图片理解` 执行详情中展示的系统提示词，变量未被替换

### MaxKB Version

2.9.2

### Problem Description

# 我设置的系统提示词：
<img width="309" height="800" alt="Image" src="https://github.com/user-attachments/assets/cf747fcb-ffce-489f-89f9-56e4bae936d6" />

# 执行详情中展示的系统提示词：
<img width="965" height="694" alt="Image" src="https://github.com/user-attachments/assets/6968d4f3-bb2c-4d4f-accb-4bc9820702f8" />