In T363669: CommunityConfiguration: eswiki beta fatals with Help panel enabled, eswiki betalabs went AWOL because GrowthExperiments schema was missing (recognized) defaults (there were defaults in the referenced schema, but not in the main schema itself). This would've caused a full site outage in production if deployment.

We should ensure all supported keys (WikiPageConfigProvider::getSupportedConfigVariableNames) are gettable via WikiPageConfigProvider::get (and ::has), across all installed providers. This is a structure test, and ideally one that would run with GrowthExperiments provided providers as well (as well as other extensions).