Update AI feature settings resolver authorization check

What does this MR do and why?

This change broadens access to the Ai::FeatureSettings::FeatureSettingsResolver resolver. A previous change restricted access to users with manage_instance_model_selection permissions only. This introduced a bug for offline instances as instance model selection is an online-only feature. The bug prevents the Model Selection page /admin/gitlab_duo/model_selection from loading correctly. This change fixes that regression by also allowing users with manage_self_hosted_models_settings permissions to access the resolver. The resolver is used by self-managed only.

References

Issue: #595107 (closed)

MR introducing regression: https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/5829

Screenshots or screen recordings

When accessing /admin/gitlab_duo/model_selection with an offline license

Before	After

Testing

Manual tests done in self-managed GDK to validate Model Selection page functionality across license and add-ons

	Duo Core	Duo Pro	Duo Enterprise	Self-hosted DAP
Premium Offline	N/A	N/A	✅	✅
Ultimate Offline	N/A	N/A	✅	✅
Premium Online	✅	✅	✅	N/A
Ultimate Online	✅	✅	✅	N/A

How to set up and validate locally

Pre-requisites:

An active Ultimate offline license.

1. Start GDK in self-managed mode GITLAB_SIMULATE_SAAS=0 gdk start
2. Run the Duo setup rake task GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup' to seed instance with a Duo Enterprise add-on.
3. Visit the Model Selection page i.e http://gdk.test:3000/admin/gitlab_duo/model_selection
4. Validate the page loads correctly as expected and looks like the "After" screenshot

MR acceptance checklist

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