Closes todos for a merge request when the MR is accepted via the API by the MR assignee.

## Are there points in the code the reviewer needs to double check?

Please review refresh service test changes to see if they are correct - I think in those cases, the todos should actually be cleared instead of left pending.

## Why was this MR needed?

To make the API behavior consistent with the UI behavior (accepting your own MRs closes the todo item and prevents them from piling up).

Closes #22477

See merge request !6486