Resolve write-in NXT fund without changing Other default

For GiveCampus integrations and API.

In Gifts to Review, enter the verified NXT fund identifier for the specific write-in gift and leave Update school level and Update gifts in batch unchecked. The resolver updates the current export row’s fund split; the school Other designation backend changes only when the school-level option is selected. Review any linked recurring row as well. Do not assign a one-off write-in fund to the shared Other definition for every donor.