Feat: Add FF for note session bar component

What does this MR do and why?

Adds the note_agent_session_bar WIP feature flag and pushes it to the frontend to both the Work Items and Merge Request page level controllers before_action.

The flag will control rendering of the note-session-bar component

Files changed:

• ee/app/controllers/ee/groups/work_items_controller.rb — pushes the new feature flag to the frontend
• ee/app/controllers/ee/projects/issues_controller.rb — pushes the new feature flag to the frontend
• ee/app/controllers/ee/projects/merge_requests_controller.rb — pushes the new feature flag to the frontend
• ee/app/controllers/ee/projects/work_items_controller.rb — pushes the new feature flag to the frontend
• ee/config/feature_flags/wip/note_agent_session_bar.yml — feature flag definition

References

• Work Item: #624326
• FF Work Item: https://gitlab.com/gitlab-org/gitlab/-/work_items/622809
• Follow-up FE MR (Add FF to Merge Request Notes)**:

Screenshots or screen recordings

Before	After
N/A (feature flag only)	N/A (feature flag only)

How to set up and validate locally

1. Enable the feature flag in rails console:

   Feature.enable(:note_agent_session_bar, current_user)

2. Visit a project's Automate page (agent sessions view).

3. Verify noteAgentSessionBar is present in the page's gon data (respectively for both Work Item and Merge Request pages.

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 #624326