Test: Refactor test fixtures and add SDK session management tests (#8141)
### What problem does this PR solve?
- Consolidate HTTP API test fixtures using batch operations
(batch_add_chunks, batch_create_chat_assistants)
- Fix fixture initialization order in clear_session_with_chat_assistants
- Add new SDK API test suite for session management
(create/delete/list/update)
### Type of change
- [x] Add test cases
- [x] Refactoring
### What problem does this PR solve?
change default models to buildin models
https://github.com/infiniflow/ragflow/issues/7774
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
Refa: HTTP API delete dataset / test cases / docs (#7657)
### What problem does this PR solve?
This PR introduces Pydantic-based validation for the delete dataset HTTP
API, improving code clarity and robustness. Key changes include:
1. Pydantic Validation
2. Error Handling
3. Test Updates
4. Documentation Updates
### Type of change
- [x] Documentation Update
- [x] Refactoring
Test: Update test cases to reduce execution time (#6470)
### What problem does this PR solve?
_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._
### Type of change
- [x] update test cases