[Bug]: vllm_ids slice error, should start from start_pos

[wuhuxiao]

https://github.com/ModelEngine-Group/unified-cache-management/blob/1d90bf358f4960e52d07b16dfd432a8e8e108b50/ucm/integration/vllm/uc_connector.py#L788C9-L788C18

[qyh111]

fixed on #250