Fix safetensors index.json overwritten by mlx-vlm regression (#624)

This model was converted with mlx-vlm v0.3.9 (mlx_vlm.convert). Due to issue #624, the correct
model.safetensors.index.json generated during conversion can be overwritten by the source model’s index,
causing an index↔shard mismatch.

Change:

Replace model.safetensors.index.json to match the actual shard files in this repo.

No weight files were changed.