[FF] ai_catalog_filter_enabled_projects -- Filter already-enabled projects
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## Summary
Feature flag gates the AI Catalog Enable modal change that disables projects where the catalog item is already enabled and shows an "Already enabled" badge, preventing the confusing "already enabled" error on submit.
- Feature MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/233159
- Feature issue: https://gitlab.com/gitlab-org/gitlab/-/issues/596801
- Flag definition:
## What the flag controls
When enabled, the modal uses the new GraphQL field to determine per-project enablement, then disables those rows in the project dropdown. When disabled, the modal falls back to the legacy project list query and allows all rows.
## Rollout notes
- Default disabled (gitlab_com_derisk).
- Enable after backend + frontend have fully deployed to avoid incompatibility.
issue