Make "merge ongoing" check more consistent

What does this MR do?

Check if there's any job running for MergeRequest#merge_jid in order to make our MR widget state more consistent.

Why was this MR needed?

Further explained on #39032 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39032 (closed)