Draft: Add loading state to edit show page

What does this MR do and why?

This MR adds a loading icon (spinning) to the Edit Page of the AI Catalog.

This makes it consistent with the loading indicator on the Run Page (see screen recording). We decided against a Skeleton Loader as this wouldn't match the designs of the form.

Screenshots or screen recordings

Screen_Recording_2025-07-24_at_10.04.10

How to set up and validate locally

1. Enable the feature flag: echo "Feature.enable(:global_ai_catalog)" | rails c
2. If needed, add some seed data: echo "FactoryBot.create_list(:ai_catalog_item, 10, item_type: :flow, project_id: 1000000)" | rails c
3. Visit http://gdk.test:3000/explore/ai-catalog/agents
4. Open an edit view by clicking on the ... menu and click on 'Edit"
5. Alternatively, open http://gdk.test:3000/explore/ai-catalog/agents/1/edit directly. It does not have to be a valid ID. Invalid IDs would make a redirect to the index page, but you'd also actually see the loading spinner for a second.
6. Go on any run page with an invalid ID, e.g. http://gdk.test:3000/explore/ai-catalog/agents/unicorn/run. Get redirected back to the index page

MR acceptance checklist

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Related to #554018 (closed) Closes #557229 (closed)