Disable AI features in sidepanel when duo_features_enabled is off

What does this MR do and why?

Part of solving: #578218

Improves the user experience when AI features are disabled (duo_features_enabled: false) in the new sidepanel design.

• All AI feature buttons now display tooltips explaining why the feature is unavailable.
• The buttons appear visually disabled with reduced opacity (gl-opacity-5) and
• The implementation uses aria-disabled instead of the disabled attribute to ensure tooltips remain functional, since disabled elements block pointer events.

demo

How to setup locally

1. Enable Duo Chat for your local Project (https://docs.gitlab.com/development/duo_agent_platform/#testing-agentic-duo-chat-in-web-ui)
2. Go to project -> settings -> General -> GitLab Duo
3. Toggle GitLab Duo off
4. See sidebar icons disabled