==== Background
`MentorWeightManager` relies on the `growthexperiments-mentorship-weight` user property only. User properties are the only place to store information about mentors with the wikitext list, however, when we switch to the structured list, we are able to store them on-wiki in a JSON blob. For that reason, when structured mentor list is enabled, `MentorWeightManager` doesn't need to be called at all.
==== Checklist
[X] {T314361}
[X] Ensure only `WikitextMentorProvider` uses `MentorWeightManager`
[X] Throw when `MentorWeightManager` is used with incorrect provider (fatal error is easier to catch than a logic error when incorrect weights are used)
==== QA notes
* Ensure that mentor weight is correctly read (in the Mentor dashboard, and ideally when mentees sign up) and written
* After wmf.25 is deployed, logstash should not have a fatal saying `MentorWeightManager cannot be used when GEMentorProvider is not "wikitext"`.