Test: Add dialog app test suite and update common.py with dialog endpoints (#8729)

### What problem does this PR solve?

This commit introduces a comprehensive test suite for the dialog app,
including tests for creating, updating, retrieving, listing, and
deleting dialogs. Additionally, the common.py file has been updated to
include necessary API endpoints and helper functions for dialog
operations.

### Type of change

- [x] Add test cases

test/testcases/test_web_api/conftest.py

@@ -21,6 +21,7 @@ from common import (
     batch_create_datasets,
     bulk_upload_documents,
     delete_chunks,
+    delete_dialogs,
     list_chunks,
     list_documents,
     list_kbs,
@@ -97,6 +98,14 @@ def clear_datasets(request: FixtureRequest, WebApiAuth: RAGFlowWebApiAuth):
     request.addfinalizer(cleanup)
 
 
+@pytest.fixture(scope="function")
+def clear_dialogs(request, WebApiAuth):
+    def cleanup():
+        delete_dialogs(WebApiAuth)
+
+    request.addfinalizer(cleanup)
+
+
 @pytest.fixture(scope="class")
 def add_dataset(request: FixtureRequest, WebApiAuth: RAGFlowWebApiAuth) -> str:
     def cleanup():