Add verification level to ai catalog items

What does this MR do and why?

Related to #572169 (closed)

This change introduces a new verification_level feature to the ai_catalog_items model. It adds a new enum column to represent how verified or trusted each AI catalog item is, with a default value of 0 (unverified).

To support efficient querying, a database index is added on the verification_level column. Additionally, a scope is defined in the AiCatalogItem model to allow filtering records based on their verification level.

How to set up and validate locally

Enable the global_ai_catalog feature flag.

1. Checkout the branch.
2. Run the database migration.
3. Confirm that existing Ai::Catalog::Item records have the default verification_level set to unverified.
4. Verify that only the values defined in ::Namespaces::VerifiedNamespace::VERIFICATION_LEVELS are allowed for the verification_level field.

DB query performance

Query:

EXPLAIN SELECT * FROM ai_catalog_items WHERE verification_level = 0

Result:

Index Scan using index_ai_catalog_items_on_verification_level on ai_catalog_items  
  (cost=0.14..2.16 rows=1 width=123) 
  (actual time=0.057..0.065 rows=12 loops=1)
  Index Cond: (verification_level = 0)
Planning Time: 0.186 ms
Execution Time: 0.102 ms