Generates a prescription discount coupon optimized for on-screen display, returned as a PNG image URL.
Supports localization — pass a locale field for non-English coupons.

Note: The drug field is optional. The live request body in the collection omits it.

Differences from /marketplace/v1/coupon/print:

• Accepts an optional locale field (BCP 47, e.g. "es-US" for Spanish)
• Returns couponURL (not printCouponURL) — image sized for screen, not print

400 error variants:

• Invalid/unknown pricingPartner → "Failed to find Contact Source for partner <name>"
• Missing or zero Quantity → structured errors object with UnitQty key
• Missing or zero price → structured errors object with Price key
• NDC not in Medispan → "Medispan drug not found for NDC: <value>"