Improve internal API for new merge request

What does this MR do?

backend : It improves internal API for new merge request widget, and reduces a technical debt.

~Frontend : Adds an if to verify if the response has pipelines key.

It seems to be the most boring solution.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27866 (closed)