NoMethodError: undefined method 'mentoring_unlocked?' for an instance of UserTrack::External (NoMethodError)

## Error

```
undefined method 'mentoring_unlocked?' for an instance of UserTrack::External (NoMethodError)

if user_track.mentoring_unlocked?
^^^^^^^^^^^^^^^^^^^^
```

## Stack trace

```
app/commands/track/search_solutions_for_mentoring.rb:20 in call
mandate/call_injector.rb:10 in Track::SearchSolutionsForMentoring.call
app/controllers/api/tracks/solutions_for_mentoring_controller.rb:4 in index
action_controller/metal/basic_implicit_render.rb:6 in send_action
abstract_controller/base.rb:224 in process_action
action_controller/metal/rendering.rb:165 in process_action
abstract_controller/callbacks.rb:259 in block in AbstractController::Callbacks#process_action
active_support/callbacks.rb:121 in block in ActiveSupport::Callbacks#run_callbacks
i18n.rb:353 in with_locale
app/controllers/application_controller.rb:57 in switch_locale!
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [NoMethodError: undefined method 'mentoring_unlocked?' for an instance of UserTrack::External (NoMethodError)](https://thalamus-ai.sentry.io/issues/103028403/)
- **Culprit**: API::Tracks::SolutionsForMentoringController#index
- **First seen:** Mar 13, 2026