When we split the merge request widget into cached and non-cached
serializers in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/15045, some merge
request endpoints (e.g. `/remove_wip`, `/cancel_auto_merge`) weren't
adapted to ensure that all the data was returned. As a result, the
frontend would set its merge request state to the partial data it did
have, but then it would clear out data that it didn't have (e.g. CI
pipeline).

When a user clicked on the `Resolve WIP Status` or `Cancel automatic
merge` button, this would cause certain elements (e.g. CI pipeline
status, merge area) to disappear until the next refresh occurred.

We worked around the problem in the MWPS refresh case on the frontend in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/26232 by forcing a
refresh, but that didn't fix the "Resolve Work in Progress" case. This
fix combines both the cached and non-cached data to ensure the response
data contains all the information necessary for the frontend.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/214750