fix Index class dropdown to only show IndexableInterface classes

[dpfaffenbauer]

Summary

The Class dropdown in the Index configuration (Studio UI) currently shows all Pimcore DataObject classes, even though only classes whose generated model implements IndexableInterface can actually be indexed. The filter logic already existed inline inside IndexController::getConfigAction, but the form-schema endpoint used an unfiltered PimcoreClassChoiceType, so the Studio dropdown still exposed every class.

This PR introduces a single source of truth for "indexable Pimcore classes" and consumes it from both the Form Schema and the Controller.

Change

• New CoreShop\Component\Index\Service\IndexableClassesProviderInterface with single method getIndexableClassNames(): array<string>.
• New CoreShop\Bundle\IndexBundle\Service\IndexableClassesProvider — walks ClassDefinition\Listing and filters by IndexableInterface (same logic that was inline in the controller).
• New CoreShop\Bundle\IndexBundle\Form\Type\IndexablePimcoreClassChoiceType — ChoiceType that consumes the provider. Used in IndexType instead of the generic PimcoreClassChoiceType.
• Refactor IndexController::getConfigAction — replaced the inline loop with a provider call.
• PimcoreClassChoiceType in ResourceBundle stays untouched (generic API for "pick any Pimcore class" remains available for other consumers).

Test plan

• Index config dropdown shows only IndexableInterface-implementing classes (CoreShopProduct, CoreShopCategory, …) — not every DataObject in the system.
• GET /pimcore-studio/api/coreshop/indexes/config returns the same filtered list as before (no behavior change on the JSON response).
• Behat / static tests pass.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
8.0% Duplication on New Code

See analysis details on SonarQube Cloud