pdf2image: update output files naming

[gi6rgi]

Description

To copy our existing ZamZar-like behaviour:

• Adjusted PDF output naming to be 0‑based;
• added filename sanitization so single‑page outputs keep the original base name while multi‑page outputs use sanitized names with page suffixes;

and simplified page normalization as a side-effect:)

Checklist

• Tests (if applicable)
• Documentation (if applicable)
• Changelog stub (or use conventional commit messages)