Update Purchase Order Fields

Operates on Purchase Orders. In the Zuper client app this module is labeled "Purchase Order" for most companies, and "Material Order" for companies in the roofing industry — the API and data shape are identical either way. Purchase Orders and Work Orders (Service Orders) share the exact same underlying schema and endpoints; they're distinguished only by the purchase_order_type field (PURCHASE_ORDER here) and the /purchase_orders vs /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. Changing vendor via this endpoint is only allowed for Work Orders (Service Orders) — attempting it on a Purchase Order / Material Order is rejected with 400.