[Doc] Improve code coverage

[RoxGamba]

The current code coverage is ~50%, as of end of April 2026. We should aim for at least 70-80%!

[RoxGamba]

Models are not currently covered by tests at all😱
As a start, we should implement the set of basic checks that is required for LVK review. For TEOBResumS, I wrote these back in the day, step zero is to clean them up and port them