test: replace ByOrganizationScope and Roles spec stubs with real tests (#5563)

[augustocbx]

What github issue is this PR for, if any?

Part of #5563 (umbrella — replaces 2 of the remaining stub specs; intentionally not using "Resolves" so the umbrella stays open)

What changed, and why?

Replaced two pending "add some tests for X" stub specs (introduced in #6517) with real tests, following the definition of done in the issue body. Both files live in spec/models/concerns/, so they are bundled as one themed PR:

• spec/models/concerns/by_organization_scope_spec.rb — exercises the .by_organization scope through CasaCase: records from another CasaOrg are excluded, and an org with no records gets an empty result. This concern is the core of the app's multi-tenancy, so it seemed worth covering first.
• spec/models/concerns/roles_spec.rb — exercises #role for all four including user types: Volunteer, Supervisor, CasaAdmin, and the separate AllCasaAdmin model.

No pending, skip, or xit remains in either file.

How is this tested? (please write rspec and jest tests!) 💖💪

This PR is tests. Commands run locally:

• bundle exec rspec spec/models/concerns/by_organization_scope_spec.rb spec/models/concerns/roles_spec.rb → 4 examples, 0 failures
• bundle exec rspec spec/models/concerns/ → 6 examples, 0 failures, 2 pending (the two remaining stub files in that directory, api_spec.rb and CasaCase/validations_spec.rb, left for follow-up PRs)
• bundle exec standardrb spec/models/concerns/ → clean

Screenshots please :)

No UI changes — test-only PR.