Ensure selectable models on dropdown are sorted

What does this MR do and why?

This MR ensures that all the models in the model selection dropdown are sorted in descending alphabetical order, improving the user experience by making it easier to find specific models.

Changes:

• Modified the listItems() computed property in ModelSelector.vue to sort selectable models by their display names

Before:

Models appeared in the order they were returned from the unit_primtives.yml config in any order, making it difficult to locate specific models.

After:

Screen_Recording_2025-10-06_at_11.26.44_AM

References

gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#1534 (closed)

How to set up and validate locally

1. Navigate to any group with AI features enabled
2. Go to the model selection interface
3. Open any model dropdown
4. Verify that models are sorted in descending alphabetical order
5. Run the frontend tests: yarn jest ee/spec/frontend/ai/model_selection/model_selector_spec.js