[ESQL] prune columns for UnionAll #138892
Description
Follow up from #137907. In the linked PR we added support for pruning columns in FORK but explicitly restricted this for FORK and not UnionAll.
We should enable it for UnionAll as well and make sure to add proper test coverage. Worth noting that removing the check in pruneColumnsInFork seems to not cause any tests to fail at the moment.
Labelling this as bug as its parent issue -> #136365.