### What problem does this PR solve?
Previously when LLM.model_name was not configured:
- System incorrectly defaulted to 'deepseek-chat' model
- This caused permission errors for unauthorized tenants
Now:
- Use tenant's default chat_model configuration first
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Previously when LLM.rerank_model was not configured:
- SDK would pass None as the value
- Database field with null=False constraint would reject it
- Caused storage failures for unset rerank_model cases
Now:
- SDK checks for None value before database operations
- Provides empty string as default when rerank_model is unset
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Add top_k for create_chat and update_chat api #4157
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
---------
Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>
### What problem does this PR solve?
Introduced [beartype](https://github.com/beartype/beartype) for runtime
type-checking.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Fix bugs in API
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
---------
Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>
