Bug: Fallback system fonts render as square if the fallback fonts are not included in the pubspec

Minimum reproduction : https://github.com/MaximeRougieux/repro_adaptive_test_datepicker_fonts

Cupertino and Material widgets use fonts that the developer might not have added in their pubspec, and thus they don't appear in the manifest and are not picked up by loadFonts.

Goldens then renders as this :

Start with the linked minimum reproduction and trace how pubspec fonts are exposed in the manifest and consumed by loadFonts during golden tests. Check the Cupertino and Material widget font paths; done means fallback fonts render as glyphs rather than squares without requiring developers to add those fonts to their pubspec.

Written by the indexing model from the issue text.