修改main分支的mlx_adapter.py中get_str_prompt函数的历史消息格式问题

[LuWei6896]

Description

When using natural language queries to search uploaded files in the chat Excel feature, the error "TypeError: can only concatenate str (not 'list') to str" occurs.

How Has This Been Tested?

Testing according to this tutorial "https://www.yuque.com/eosphoros/dbgpt-docs/usoemkv2qddbyl33"。

Snapshots:

Checklist:

• [ * ] My code follows the style guidelines of this project
• [ * ] I have already rebased the commits and make the commit message conform to the project standard.
• [ * ] I have performed a self-review of my own code
• [ * ] I have commented my code, particularly in hard-to-understand areas
• [ * ] I have made corresponding changes to the documentation
• [ * ] Any dependent changes have been merged and published in downstream modules