Backport '595107/fix-model-selection-ui-regression' into 18.8

What does this MR do and why?

Backports !229962 (merged) into 18.8 to fix a product regression.

Details of change

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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