Duo Self-Hosted: Add GitLab managed models as options in model selector dropdown

What does this MR do and why?

This MR makes updates to the Duo Self-Hosted model selector component to support GitLab managed models offered by instance-level model selection. This will replace the existing GitLab AI vendor model option for Duo Self-Hosted as part of hybrid configuration.

However, we will still need to support it at this stage until the instance-level model selection feature flag is fully rolled out and the documentation is updated.

Note: the following changes will be handled separately:

• GitLab default model option
• Bulk update

References

Update Duo Self-Hosted model selector dropdown ... (#565365 - closed)

Screenshots or screen recordings

Self-hosted models only	Self-hosted and GitLab managed models
self_hosted	self_hosted_and_gitlab_managed

How to set up and validate locally

Prerequisites:

• Run GDK on non-SASS mode: GITLAB_SIMULATE_SAAS=0 gdk start
• Run the following setup script to get Duo Enterprise add on: GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup'
• Access Duo Self-Hosted feature settings: /admin/gitlab_duo/self_hosted/features.
• Create a self-hosted model if you haven't already.

Self-hosted only:

• Disable the instance_level_model_selection feature flag.
• Access Duo Self-Hosted feature settings: /admin/gitlab_duo/self_hosted/features
• In the dropdown, note that only self-hosted models are present (no GitLab managed models)
• Observe that GitLab AI vendor model option appears in the list.
• Observe that disabled option appears as a separate group, no longer part of Self-Hosted models
• Ensure model selection works as expected

Self-hosted and GitLab managed models:

• Enable the instance_level_model_selection feature flag.
• Access Duo Self-Hosted feature settings: /admin/gitlab_duo/self_hosted/features
• In the dropdown, note the presence of GitLab managed models in addition to self-hosted models
• Observe that disabled option appears as a separate group, no longer part of Self-Hosted models
• Ensure model selection works as expected

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