Add duo_panel_session_inbox feature flag

What does this MR do and why?

Adds the duo_panel_session_inbox feature flag (type: wip, default off) and pushes it to the frontend via push_frontend_feature_flag in ee/lib/ee/gitlab/gon_helper.rb. The flag has to be in gon because the Duo side panel's Vue router reads window.gon.features.duoPanelSessionInbox when it initialises. Nothing branches on the flag yet; the component changes that make the Sessions route render a tabbed inbox instead of the current flat list come in a follow-up MR.

References

• Feature issue: #616493
• Rollout issue: #616496
• Parent epic: gitlab-org#23179

Screenshots or screen recordings

No UI change in this MR.

How to set up and validate locally

1. In a Rails console, enable the flag:

   Feature.enable(:duo_panel_session_inbox)

2. Reload any page and open the browser console.
3. Confirm gon.features.duoPanelSessionInbox is true. There is no visible change to the panel; that is expected.

MR acceptance checklist

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