fix: add default value for response parser.

[jiaoqiyuan]

Description

fix #2158

In the ChatDB mode, when performing database Q&A based on the DeepSeek model, the returned data contains SQL but is not correctly parsed due to the absence of the direct_response field. Here, we set default empty string values for the display_type and direct_response fields to avoid situations where the large model cannot fully return as required (but still returns SQL), leading to an inability to answer questions.

How Has This Been Tested?

• make fmt
• make mypy
• make test

Snapshots:

Response: Can not find sql in response：

It is because the returned JSON content includes thoughts, sql, and direct_response, but is missing display_type.

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