Model Selection: Enable model access controls for internal team members only

Problem

Model Selection streamlines the process of making new models available to users for quick adoption. The trade-off is that this approach eliminates our previous ability to gate specific models behind feature flags, a process we often used in the past for testing and gradual rollouts of new models.

Proposal

We can enhance our YAML-based configuration with a new beta property that restricts model visibility and selection exclusively to GitLab team members (those in the gitlab-org group). This approach enables us to add new models, deploy changes, and conduct internal testing without exposing these experimental models to our broader customer base.