As part of the Structured mentor list project, we added support for warnings to the config validators. The warnings were added, because regular validation errors makes GrowthExperiments to fully ignore the config file. Warnings, on the other hand, means that the config file is still used, while complaining to users that something is wrong. In other words, warning means "don't allow introducing this but don't fail if it's already present".
It turns out that the ConfigHooks::EditFilterMergedContent hook ignores warnings, while it actually shouldn't do that. This means that mentors can add a too long message via Special:EnrollAsMentor or their mentor dashboard, but admins can still do that accidentally by making a manual edit to MediaWiki:GrowthMentors.json (ie. the underlying storage). Let's fix that by making ConfigHooks::EditFilterMergedContent respect the warnings.
See conversation at T314691#8145251 et seq. for context.
 Structured mentor list and community configuration share a significant portion of logic (we treat the list of mentors as a configuration, basically).
- When an admin adds a message longer than 240 characters via a manual edit to MediaWiki:GrowthMentors.json, the edit is rejected.