featurize_files ignores single_roi=True for whole-slide images

<img width="1017" height="421" alt="Image" src="https://github.com/user-attachments/assets/b7340fad-7263-47e8-8a46-6b8ac14be69f" />

When using the featurize_files function in nyxus with single_roi=True, the flag does not appear to be respected for whole-slide images. Instead of computing features for a single ROI representing the entire image