Fix: fixed invalid save() arguments for slide thumbnails (#8851)

### What problem does this PR solve?

Fixed invalid save() arguments for slide thumbnails.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/app/presentation.py

                     with BytesIO() as buffered:
                         slide.get_thumbnail(
                             0.5, 0.5).save(
-                            buffered, drawing.imaging.ImageFormat.jpeg, quality=80)
-                        imgs.append(Image.open(buffered))
+                            buffered, drawing.imaging.ImageFormat.jpeg)
+                        buffered.seek(0)
+                        imgs.append(Image.open(buffered).copy())
                 except RuntimeError as e:
                     raise RuntimeError(f'ppt parse error at page {i+1}, original error: {str(e)}') from e
         assert len(imgs) == len(

rag/svr/task_executor.py

                         d["image"].close()  # Close original image
                         d["image"] = converted_image
                     d["image"].save(output_buffer, format='JPEG')
-                
+
                 async with minio_limiter:
                     await trio.to_thread.run_sync(lambda: STORAGE_IMPL.put(task["kb_id"], d["id"], output_buffer.getvalue()))
                 d["img_id"] = "{}-{}".format(task["kb_id"], d["id"])
                     nursery.start_soon(delete_image, task_dataset_id, chunk_id)
             progress_callback(-1, msg=f"Chunk updates failed since task {task['id']} is unknown.")
             return
-        
+
     logging.info("Indexing doc({}), page({}-{}), chunks({}), elapsed: {:.2f}".format(task_document_name, task_from_page,
                                                                                      task_to_page, len(chunks),
                                                                                      timer() - start_ts))
         finally:
             redis_lock.release()
         await trio.sleep(30)
-        
-        
+
+
 async def task_manager():
     try:
         await handle_task()