Update Work Order Fields

Operates on Service Orders, shown in the Zuper client app as "Work Order" (in every industry). Work Orders share the exact same underlying schema and endpoints as Purchase Orders / Material Orders; they're distinguished only by the purchase_order_type field (SERVICE_ORDER here) and the /service_orders route prefix. This is a narrower field set than the full PUT update — intended for lightweight edits (title, dates, remarks, vendor/payment term) without resending the whole order. Work Orders (Service Orders) may change vendor (the assigned subcontractor) through this endpoint.