* Refactor: add pure pdf_ops module with real-PDF tests
* Drops tests we don't relly need + adds the typing annotation
* Now that I'm sure it works, re-work these tests to mock pdf_ops, rather than pikepdf
* Validates pages ahead of time in build_pdfs rather than during the building
* Adds the given/when/then and some additional cleanups of pytest style
* This is redundant now
