Fix trainer adapter save fallback

[Blaizzy]

Summary

• Resolve a fallback adapter save path when trainer args provide adapter_file=None
• Apply the same resolved path to checkpoint and final saves in SFT and ORPO trainers
• Add a regression test for the SFT save path

Fixes #908.

Validation

• uv run --with pytest pytest mlx_vlm/tests/test_trainer.py -q
• commit hooks: black, isort, autoflake