Hide GitLab managed models from the UI for CLI feature setting as they don't work

Description

When instance-level model selection is enabled, the GitLab Duo CLI feature setting won't work with any GitLab managed/vendored models right now until we provide a fix for it. But we're still surfacing these models in the UI dropdown and it is causing a buggy/confusing experience. For now, a simple solution is to hide these models until we can get around to implementing the fix above.

Definition of Done

• The GitLab AI Vendor option is always hidden for GitLab Duo CLI
• GitLab managed models are always hidden for GitLab Duo CLI