Add message search and overflow menu for commit list header

What does this MR do and why?

(summary generated by AI)

This merge request enhances the commits page in a GitLab project with several improvements:

1. Added a new search filter - Users can now filter commits by message content, making it easier to find specific commits by searching within their descriptions.
2. Added an actions menu - A new dropdown menu (three dots icon) appears in the top-right corner of the commits page, providing quick access to:
   • Browse files at the current commit
   • Access the commits RSS feed
3. Improved search experience - The search bar now shows more user-friendly text and treats search terms as proper filter tokens, making the interface cleaner and more intuitive.

References

Finish state of the header

Implementation Plan

Task	MR
Add ref selector	!200567 (merged)
Add breadcrumb	follow up
Add overflow menu	👈 this MR
Add filter search and support author search	!199810 (merged)
Add support for message search	👈 this MR

Screenshots or screen recordings

CleanShot 2025-08-06 at 23.06.47.mp4

Viewport size	Screenshot
xs (<576px)
sm (>=576px)

How to set up and validate locally

1. Enable project_commits_refactor FF
2. Open a project
3. Code > Commits

⚠️ the list of commits uses mock data and would not change when ref gets updated. to validate this change, i confirm the link generated from the dropdown items matches the existing one (when FF is off) 👍

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 #547682 (closed)