Import preprocessor classes from the dynamically loaded NODE_CLASS_MAPPINGS
dictionary rather than trying to import directly from module paths.
This matches how comfyui_controlnet_aux actually exports its classes.
Verified correct node names:
- DepthAnythingV2Preprocessor ✓
- DWPreprocessor ✓
- CannyEdgePreprocessor ✓
- OpenposePreprocessor ✓
- MiDaS-DepthMapPreprocessor ✓