Add session details overlay to the Duo side panel

What does this MR do and why?

• Adds a collapsible "Session details" bar pinned to the bottom of the Duo side panel on session details, behind a new session_details_side_panel_info_overlay feature flag
• Collapsed state shows a title and a summary of available fields
• Expanded state shows the session ID, the flow and its version, CI job links, and the default model
• Replaces the header "Information" button and its collapsible region in the side panel
• The full page (non side panel) session view is unchanged

References

• Related to Sessions details panel redesign (side panel vie... (#609576 - closed) • Allison Villa • 19.4
• FF rollout issue: [FF] Roll out `session_details_side_panel_info_... (#624497) • Allison Villa • 19.5
• Design (side panel): https://claude.ai/design/p/2f715efd-fdb9-46c6-9737-3ca61ec15a5f?file=Agent+Session+Drawer.dc.html

Screenshots or screen recordings

Collapsed	Expanded

Stays at the bottom when session details are short

Collapsed	Expanded

How to set up and validate locally

1. Prerequisite: Have GitLab Duo set up in your GDK
   • Have a Duo-enabled group and project in your GDK created by the Duo setup above
2. Prerequisite: Have GitLab Runner working
3. Enable the session_details_side_panel_info_overlay feature flag
4. Start a new session or view an existing one, then view the session in the Duo side panel
5. See that the new information overlay is rendered at the bottom of the panel and that expanding it shows the session ID, the flow that ran and its version, CI job, and the default model

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.