Test: Refactor test concurrency handling and add SDK chunk management tests (#8112)

### What problem does this PR solve?

- Improve concurrent test cases by using as_completed for better
reliability
- Rename variables for clarity (chunk_num -> count)
- Add new SDK API test suite for chunk management operations
- Update HTTP API tests with consistent concurrency patterns

### Type of change

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

test/testcases/test_sdk_api/common.py

@@ -22,11 +22,7 @@ from utils.file_utils import create_txt_file
 
 # DATASET MANAGEMENT
 def batch_create_datasets(client: RAGFlow, num: int) -> list[DataSet]:
-    datasets = []
-    for i in range(num):
-        dataset = client.create_dataset(name=f"dataset_{i}")
-        datasets.append(dataset)
-    return datasets
+    return [client.create_dataset(name=f"dataset_{i}") for i in range(num)]
 
 
 # FILE MANAGEMENT WITHIN DATASET
@@ -39,3 +35,8 @@ def bulk_upload_documents(dataset: DataSet, num: int, tmp_path: Path) -> list[Do
         document_infos.append({"display_name": fp.name, "blob": blob})
 
     return dataset.upload_documents(document_infos)
+
+
+# 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)]