feat: Add Duo CLI admin application setting (backend)

What does this MR do and why?

Adds the duo_cli_enabled as a new instance-level AI setting to allow admins to disallow use of the non-headless Duo CLI. Puts it behind a feature flag as requested by the PM. Updates the POST /api/v4/ai/duo_workflows/workflows endpoint to check whether the client is a non-headless Duo CLI, in which case if duo_cli_enabled is false, the client receives a 403 Forbidden error (which it will handle similarly to other errors due to failed access; see [https://gitlab.com/gitlab-org/editor-extensions/gitlab-lsp/-/merge_requests/3313](feat(cli): gate Duo CLI behind duo_cli_enabled admin setting). Otherwise, existing Duo CLI behavior is unchanged.

Changes:

• Add duo_cli_enabled_setting feature flag (disabled by default for 19.0)
• Add duo_cli_enabled as a jsonb-backed field to the ai_settings table (default: true)
• Enforce at POST /api/v4/ai/duo_workflows/workflows: returns 403 when requires_duo_cli_enabled is true and duo_cli_enabled is false, since every CLI session calls this endpoint at startup

Database query

UPDATE (.update!(duo_cli_enabled: value)):

UPDATE "ai_settings"
SET "updated_at" = NOW(), "feature_settings" = '{"duo_agent_platform_enabled":true,"duo_cli_enabled":true}'
WHERE "ai_settings"."id" = 1

Query plan:

Update on ai_settings  (cost=0.15..2.17 rows=0 width=0) (actual time=0.113..0.113 rows=0 loops=1)
  Buffers: shared hit=5
  ->  Index Scan using ai_settings_pkey on ai_settings  (cost=0.15..2.17 rows=1 width=46) (actual time=0.047..0.048 rows=1 loops=1)
        Index Cond: (id = 1)
        Buffers: shared hit=2
Planning Time: 0.157 ms
Execution Time: 0.134 ms

References

• Implements: #597833 (closed)
• LSP MR: [https://gitlab.com/gitlab-org/editor-extensions/gitlab-lsp/-/merge_requests/3313](feat(cli): gate Duo CLI behind duo_cli_enabled admin setting)

How to set up and validate locally

1. Start GDK
2. Enable the feature flag: Feature.enable(:duo_cli_enabled_setting)
3. In Rails console:
   • Check default (should be true): Ai::Setting.instance.duo_cli_enabled
   • Change the setting: Ai::Setting.instance.update!(duo_cli_enabled: false)
4. Make a workflow creation request with requires_duo_cli_enabled: true in the body. With duo_cli_enabled false, it should return 403:

   curl -X POST "http://gdk.test:3000/api/v4/ai/duo_workflows/workflows" \
     -H "Authorization: Bearer <token>" \
     -H "Content-Type: application/json" \
     -d '{"requires_duo_cli_enabled": true}'

5. With requires_duo_cli_enabled true (or the feature flag off), the same request should return a workflow response object with no error. You should be able to point Duo at your GDK (from within a project set up with Duo on your local instance) and have it work normally.

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