Test: Add web API test suite for knowledge base operations (#8254)

### What problem does this PR solve?

- Implement RAGFlowWebApiAuth class for web API authentication
- Add comprehensive test cases for KB CRUD operations
- Set up common fixtures and utilities in conftest.py
- Add helper functions in common.py for web API requests

The changes establish a complete testing framework for knowledge base
management via web API endpoints.

### Type of change

- [x] Add test case

test/testcases/test_sdk_api/conftest.py

@@ -116,7 +116,7 @@ def clear_session_with_chat_assistants(request, add_chat_assistants):
 
 
 @pytest.fixture(scope="class")
-def add_dataset(request: FixtureRequest, client: RAGFlow):
+def add_dataset(request: FixtureRequest, client: RAGFlow) -> DataSet:
     def cleanup():
         client.delete_datasets(ids=None)