Update Duo Self-Hosted model selector dropdown to display instance-level models

Description

Model Selection for self-managed instances are only enabled if they are also using the cloud-connected AIGW. The goal of this issue is to update the Duo Self-Hosted model selector dropdown to support instance-level model selection.

Mockup thread: #557247 (comment 2663608380)+

TODO:

• Remove the GitLab AI vendor model option, as this will be replaced with the GitLab default model option provided by the instance setting.
• Update the Duo Self-Hosted model selector component to display GitLab-hosted models provided by instance-level model selection when user has access to CC.
• Don't display the GitLab-hosted models as options for DAP features
• Ensure batch update button works as expected