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_http_api/test_dataset_mangement/test_delete_datasets.py

@@ -99,14 +99,14 @@ class TestCapability:
 class TestDatasetsDelete:
     @pytest.mark.p1
     @pytest.mark.parametrize(
-        "func, expected_code, expected_message, remaining",
+        "func, expected_code, remaining",
         [
-            (lambda r: {"ids": r[:1]}, 0, "", 2),
-            (lambda r: {"ids": r}, 0, "", 0),
+            (lambda r: {"ids": r[:1]}, 0, 2),
+            (lambda r: {"ids": r}, 0, 0),
         ],
         ids=["single_dataset", "multiple_datasets"],
     )
-    def test_ids(self, HttpApiAuth, add_datasets_func, func, expected_code, expected_message, remaining):
+    def test_ids(self, HttpApiAuth, add_datasets_func, func, expected_code, remaining):
         dataset_ids = add_datasets_func
         if callable(func):
             payload = func(dataset_ids)