fix: remove trailing comma from debug MPO JSON

[Travor278]

Description

Removes the trailing comma from internvl_chat_gpt_oss/shell/data/debug_mpo.json so the file is valid JSON again.

This only changes the debug MPO metadata file syntax. It does not change any training script defaults or dataset entries.

Related Issue

Resolves #1265.

Motivation and Context

The MPO shell scripts pass shell/data/debug_mpo.json as --meta_path. Loading that file with Python's json module currently raises JSONDecodeError because standard JSON does not allow a trailing comma after the final object entry.

How Has This Been / Can This Be Tested?

Tested locally on Windows with Anaconda py311 / Python 3.11.15.

D:\Dev\conda-envs\py311\python.exe -m json.tool internvl_chat_gpt_oss\shell\data\debug_mpo.json
D:\Dev\conda-envs\py311\python.exe -m json.tool internvl_chat_gpt_oss\shell\data\debug_sft.json

Both JSON files parse successfully.

Checklist

• Removed the invalid trailing comma from debug_mpo.json.
• Verified the edited JSON file with Python's standard JSON parser.
• Left training scripts and dataset metadata values unchanged.