fix: only delete individual LoRA file instead of entire parent directory

[Pfannkuchensack]

Summary

When deleting a file-based model (e.g. LoRA), the previous logic used rmtree on the parent directory, which would delete all files in that folder — even unrelated ones. Now only the specific model file is removed, and the parent directory is cleaned up only if empty afterward.

QA Instructions

1. Place multiple LoRA .safetensors files in the same directory under the models folder
2. Register both as models in InvokeAI (in-place)
3. Delete one of them via the UI
4. Verify that only the deleted model's file is removed, and the other file remains intact
5. Delete the second model and verify the now-empty directory is cleaned up

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[lstein]

Looks good.