### What problem does this PR solve?

This PR introduces Pydantic-based validation for the update dataset HTTP
API, improving code clarity and robustness. Key changes include:
1. Pydantic Validation
2. ​​Error Handling
3. Test Updates
4. Documentation Updates
5. fix bug: #5915

### Type of change

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

### What problem does this PR solve?

cover [List chat assistant's
sessions](https://ragflow.io/docs/dev/http_api_reference#list-chat-assistants-sessions)
endpoints

### Type of change

- [x] Update test cases

### What problem does this PR solve?

cover [parse
documents](https://ragflow.io/docs/dev/http_api_reference#parse-documents)
endpoints

### Type of change

- [x] add test cases

### What problem does this PR solve?

cover [download docments
endpoints](https://ragflow.io/docs/dev/http_api_reference#download-document)

### Type of change

- [x] add test cases

### What problem does this PR solve?

cover upload docments endpoints

### Type of change

- [x] add test cases