Add visibility levels to agent form

What does this MR do and why?

Add visibility settings to agent form

Private for users with projects access, and public for any user without authorization.

Screenshots or screen recordings

Screen_Recording_2025-07-21_at_17.16.16

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. Create a private agent
5. Create a public agent - shows an info box
6. Edit the private one - shows an info box on public (same as 5)
7. Edit the public one - shows an info box on private

MR acceptance checklist

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Related to #555081 (closed)