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

test/testcases/test_sdk_api/common.py

@@ -16,7 +16,7 @@
 
 from pathlib import Path
 
-from ragflow_sdk import DataSet, Document, RAGFlow
+from ragflow_sdk import Chat, Chunk, DataSet, Document, RAGFlow, Session
 from utils.file_utils import create_txt_file
 
 
@@ -38,10 +38,15 @@ def bulk_upload_documents(dataset: DataSet, num: int, tmp_path: Path) -> list[Do
 
 
 # CHUNK MANAGEMENT WITHIN DATASET
-def batch_add_chunks(document: Document, num: int):
+def batch_add_chunks(document: Document, num: int) -> list[Chunk]:
     return [document.add_chunk(content=f"chunk test {i}") for i in range(num)]
 
 
 # CHAT ASSISTANT MANAGEMENT
-def batch_create_chat_assistants(client: RAGFlow, num: int):
+def batch_create_chat_assistants(client: RAGFlow, num: int) -> list[Chat]:
     return [client.create_chat(name=f"test_chat_assistant_{i}") for i in range(num)]
+
+
+# SESSION MANAGEMENT
+def batch_add_sessions_with_chat_assistant(chat_assistant: Chat, num) -> list[Session]:
+    return [chat_assistant.create_session(name=f"session_with_chat_assistant_{i}") for i in range(num)]