Feat: update visuals of agent sessions list page

What does this MR do and why?

This MR updates the Agent Sessions page to a more simplified design that can be reutilized in the new Side Panel.

Related Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/560590

References

Screenshots or screen recordings

Before	After	Screen Recording/Behavior
		Screen_Recording_2025-08-27_at_4.42.40_PM

How to set up and validate locally

1. Make sure you have AI features enabled https://docs.gitlab.com/development/ai_features/
2. Turn on the following feature flags: duo_workflow, duo_workflow_in_ci, and ai_flow_triggers
3. Navigate to a project with that has AI features enabled (see Step 1).
4. In the left side panel, you will see Automate with a tanuki icon.
5. Click the dropdown and select Agent sessions. This will load your agent sessions in that project
6. If you need to populate flows, run the rake task for your project (ee/lib/tasks/gitlab/duo_workflow/duo_workflow.rake) bundle exec rake "gitlab:duo_workflow:populate[50,20,1,gitlab-duo/test]".
7. Validate session items have the appropriate labeling, status and icon.
8. Hover over a session item and notice the hover state matches the design.
9. Click on a session and validate it still re-routes.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #560590