Use returnable: false in the widgets test schema

[jwils]

Summary

Use returnable: false in the widgets test schema so the feature is exercised by the stock schema and generated artifacts.

This branch adds:

• a hidden leaf field on Widget: internal_name
• a hidden embedded object on Widget: internal_details.name
• matching widget factory data
• regenerated schema artifacts (with and without Apollo)

Testing

• bundle exec rake schema_artifacts:dump

Stacked on top of #1108.