Chat Empty State - Duo is Off First Iteration

Problem

When GitLab Duo Agentic Mode is currently unavailable (neither enabled nor accessible), users need a clear interface that explains the unavailability and provides guidance on how administrators can enable the feature.

Note: We allow users to toggle back to Classic Chat if Classic chat is available.

Design Proposal

Note: This is the first iteration before we can better distinguish why Duo is unavailable and give more specialized advice.