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/libs/auth.py

@@ -23,3 +23,12 @@ class RAGFlowHttpApiAuth(AuthBase):
     def __call__(self, r):
         r.headers["Authorization"] = f"Bearer {self._token}"
         return r
+
+
+class RAGFlowWebApiAuth(AuthBase):
+    def __init__(self, token):
+        self._token = token
+
+    def __call__(self, r):
+        r.headers["Authorization"] = self._token
+        return r