Feat: prevent agency from being saved if it doesn't have copy for index

[angela-tran]

While working on #2705, I realized that it would help prevent errors if TransitAgency.clean checks that the agency has all the copy it will need. That is, the clean method should try to call each @property method for getting the context dictionary for whatever page and make sure calling that method doesn't raise an error.

This is similar to how EnrollmentFlow.clean checks that the flow is able to get its in_person_eligibility_context if it is an in-person flow.

Basically if somehow we've forgotten to add copy for some slug, or if we accidentally remove copy from one of the mappings, then upon saving, we'll realize it.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
benefits/core/models
transit.py
Project Total

This report was generated by python-coverage-comment-action

[angela-tran]

I realized while writing the PR description that this doesn't fully protect us from errors. For example, if we removed an entry for an existing TransitAgency, an error could still happen.

It would be better to have unit tests that can be more comprehensive.
(A unit test could iterate over the AgencySlug values and check that the core_context.agency_index mapping contains all of them. Something like that.)