Improve AutoImageProcessor error for unavailable backends
Author: sisaman
Pull request: yes

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Fixes #46726

Improves the `AutoImageProcessor.from_pretrained` error when a model/image processor mapping is recognized, but none of the mapped backend implementations can be imported.

Instead of falling through to a generic or misleading image processor resolution error, the new message lists the resolved backend classes and calls out missing optional dependencies such as `torchvision` or `Pillow`.

### Why

When optional vision backends are unavailable, users can see errors that imply the checkpoint/config is unrecognized even though the actual issue is a missing backend dependency. This is especially confusing for processors whose available implementation depends on `torchvision`.

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Comments:
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