Remove session_details_side_panel_info_overlay feature flag

What does this MR do and why?

• Removes the session_details_side_panel_info_overlay feature flag. This replaces the information icon in the panel header and its collapsible section with a "Session details" bar pinned to the bottom of the panel.
• Also removes related code that only existed for the old information toggle and section
  • The information button and its props in ai_panel.vue / content_container.vue
  • The gl-collapse branch and the maximized-panel layout variant in agent_flow_details.vue. With the collapse gone the wrapper has a single child in side-panel view, so the row-reverse layout no longer had any effect
  • The isSidePanelView prop and its styling branches in agent_flow_info.vue, which now renders only in the page view
  • The unused setInfoToggleVisible helper in ee/ai/graphql

References

• Related to [FF] Roll out `session_details_side_panel_info_... (#624497 - closed) • Allison Villa • 19.5 and Session Details: Remove the old info toggle (#627222 - closed) • Allison Villa
• MR that added the feature: Add session details overlay to the Duo side panel (!252388 - merged) • Allison Villa • 19.4

Screenshots or screen recordings

Before	After

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. Open the sessions list in the Duo side panel and click on a session
4. Confirm that you see the session details overlay bar and that clicking on it expands the details

MR acceptance checklist

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