Add Rails support for default Duo namespace assignment selection | Model Selection

Context

For Model selection when no namespace is given because a developer is working in a new repo that is not yet an GitLab project or because they use the various AI capabilities of the GitLab CLI. We are routing the request to a default duo enabled namespace chosen by the user.

Acceptance Criteria

We need to pass the following via Rails:

• Add a new helper method to return the eligible Duo namespace assignment options
• Pass the default_duo_add_on_assignment_id param to the user preferences controller.

This is done instead of creating GQL queries/mutations as the user preferences page already adopt the Rails pattern to pass and submit data.