Align async HF non-weight file export

[caixianzhang]

Summary

• Align async HF non-weight file export with sync HF by preferring generated HF config when available.
• Keep fallback copying from the original HF path when no hf_config is available.

Verification

• python3 -m py_compile xtuner/v1/model/base.py
• Compared async/sync HF non-safetensors outputs; config.json and model.safetensors.index.json now match by sha256.