Merging dev broke this branch: SH-126/SH-184 made Add-On a backend-authoritative
indicator (isAddOn) and removed it from the WOType union, but
read-detail-closability-fields still accepted "Add-On" and mapped "AddOn" onto
it, so tsc failed on both the mapper and the closability test.
Drop Add-On from the detail type set so a legacy payload normalizes to "", and
cover the normalization directly instead of asserting it through
detailToClosabilityInput, which only forwards the already-typed value.