Stripe React Native PaymentSheet has no promotional code or coupon entry field

Stripe's React Native SDK offers a pre-built `PaymentSheet` UI that handles card entry and payment confirmation. Mobile developers using it cannot add a promotional code or coupon redemption field to that same sheet. Stripe's Checkout Sessions (web) support `allow_promotion_codes` and `discounts`, but the React Native `PaymentSheet` exposes no equivalent. The Stripe team responded to the feature request in 2025 stating it is 'not planned yet' and suggested developers handle it outside the PaymentSheet, which forces them to build their own checkout UI from scratch -- losing the PCI scope reduction benefit of PaymentSheet. Developers need a wrapper or SDK extension that adds a promo code input that validates against the Stripe API and applies the discount before the payment confirmation step, all within the PaymentSheet session.