fix: invalid image page_range crashes with AttributeError instead of clear error

[YizukiAme]

Fixes #1019

Root cause
ImageProvider validated page_range with an assert, but the assertion message referenced self.doc, which does not exist for image inputs. Out-of-range values therefore crashed with AttributeError while formatting the message instead of surfacing a normal validation error.

Fix
This change replaces the assertion with an explicit ValueError that uses self.image_count for the bounds message and adds a regression test for an invalid single-image page_range.

[github-actions]

CLA Assistant Lite bot:
Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request}

[YizukiAme]

recheck