### 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 license statement.

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

### 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>

### What problem does this PR solve?

Manage ragflow-sdk with poetry
### Type of change

- [x] Refactoring

### What problem does this PR solve?

Fix bugs in API


- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Turn resource to plural form

### Type of change
- [x] Refactoring

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Fix some issues in API and test

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Fix some issues in API

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Fix some issues in API

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Refactor API for document and session.

### Type of change


- [x] Refactoring

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Refactor Chat API

### Type of change

- [x] Refactoring

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Fix: renrank_model and pdf_parser bugs | Update: session API
#2575
#2559
### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Refactoring

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

### What problem does this PR solve?

Includes SDK for creating, updating sessions, getting sessions, listing
sessions, and dialogues
#1102 
### 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?

SDK for session
#1102 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Feiue <10215101452@stu.ecun.edu.cn>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

### What problem does this PR solve?

SDK for Assistant
#1102 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

Co-authored-by: Feiue <10215101452@stu.ecun.edu.cn>