Test: Add SDK API tests for chat assistant management and improve con… (#8131)

### What problem does this PR solve?

- Implement new SDK API test cases for chat assistant CRUD operations
- Enhance HTTP API concurrent tests to use as_completed for better
reliability

### Type of change

- [x] Add test cases
- [x] Refactoring

test/testcases/test_sdk_api/common.py

@@ -40,3 +40,8 @@ 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):
     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):
+    return [client.create_chat(name=f"test_chat_assistant_{i}") for i in range(num)]