Update Duo Self-hosted URL Routing

What does this MR do and why?

Moves existing Duo self-hosted routing from /admin/ai/duo_self_hosted/ to /admin/gitlab_duo/self_hosted/. This change concludes the move of Duo self-hosted to the admin instance AI settings page.

References

GitLab Duo Self-hosted

Screenshots or screen recordings

URL update only - no other visual changes

Before	After

How to set up and validate locally

Pre-requisites

• An active online or offline, Ultimate or Premium license.

1. Start GDK in self-managed mode GITLAB_SIMULATE_SAAS=0 gdk start
2. Run the Duo Enterprise setup rake task GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup[duo_enterprise]' to seed DB with enterprise add-on purchase
3. Click to Admin area (button on left-hand sidebar)
4. Navigate to GitLab Duo and click "Configure GitLab Duo Self-hosted". This should navigate to http://gdk.test:3000/admin/gitlab_duo/self_hosted.

Validation:

All pages should work under new routing:

• Index: /admin/gitlab_duo/self_hosted
• AI Features tab: /admin/gitlab_duo/self_hosted/features
• Self-hosted models tab: /admin/gitlab_duo/self_hosted/models
• New self-hosted model page: /admin/gitlab_duo/self_hosted/model/new
• Edit self-hosted model page: /admin/gitlab_duo/self_hosted/model/:id/edit

Ensure that:

• Tab navigation in /admin/gitlab_duo/self_hosted works as expected
• You can create a self-hosted model
• You can edit a self-hosted model
• You can open any feature settings dropdown and click "Add self-hosted model" to be redirected to the new model page
• The old /admin/ai/duo_self_hosted/ path should redirect to new path

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #524054 (closed)