Add ai_catalog_synthetic_foundational_items feature flag

What does this MR do and why?

Adds the ai_catalog_synthetic_foundational_items feature flag (type: wip, default: false).

This feature flag will gate the loading of AI Catalog foundational items as synthetic SQL rows from Ai::FoundationalChatAgent FixedItemsModel definitions, instead of storing them in the database. This approach:

• Works automatically on Self-Managed, Dedicated, and Cells without seeding
• Works out of the box in GDK
• Allows foundational items to appear in the explore page with native filtering and pagination

This MR only adds the feature flag definition. The implementation will follow in subsequent MRs.

Related

• Feature issue: #592038
• Rollout issue: #595053
• Draft spike MR: !226723

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