Hide Model Selection menus when Amazon Q is enabled

There is no need for anything model selection feature when AmazonQ is enabled for a an instance. Therefore we need to hide the following menus:

• Model Selection user default namespace: http://gdk.test:3000/-/profile/preferences
• Model Selection menu: http://gdk.test:3000/groups/gitlab-duo/-/settings/gitlab_duo/model_selection

Implementation proposal:

These menu apperance in the page are ruled by policies. Find out what policies they use and add ::Ai::AmazonQ.connected? clause to them.

You can change the following policies to achieve this:

• :manage_instance_model_selection at https://gitlab.com/gitlab-org/gitlab/blob/bfabf6c5c02222d58881b8dfbfd9081905d3a39f/ee/app/policies/ee/global_policy.rb#L196
• :admin_group_model_selection at https://gitlab.com/gitlab-org/gitlab/blob/6fc44432f79a350f2d732c2f67f9d8138e491e7e/ee/app/policies/ee/group_policy.rb#L1078
• :assign_default_duo_group at https://gitlab.com/gitlab-org/gitlab/blob/c2ef01544ec5ea8b8f74766bc2c982db2fc1db2c/ee/app/policies/ee/user_policy.rb#L51