Validate info.json Configuration Early

As it stands, the system only checks for the resources it needs to render, which means that the system will sometimes err only when Save & Grade is hit due to ./solution being missing or showCorrectAnswer being un-set.

It is worth validating the full JSON config before rendering to keep the "once a component appears to work, it works" philosophy in place like the rest of PL.

Start at the code path that handles render and Save & Grade and currently checks only required resources. Locate where `info.json` is loaded and introduce/extend pre-render validation so missing fields (for example `showCorrectAnswer`) and required resources like `./solution` are rejected before rendering. Then run the relevant render/Save & Grade tests or checks and confirm invalid configs now fail early with a clear error.

Scritto dal modello di indicizzazione a partire dal testo della issue.