Show an Apply discount field on the inline payment form (confirmed sessions)

Summary

Let customers enter and apply a discount code directly on the inline payment form, alongside the card details — not only on the full/overlay checkout.

Current behaviour

feature_flags.allow_discount_code renders the coupon UI on the standard checkout flow only. On a confirmed session (confirm: true), the inline form renders just the payment fields and the pay button, so there is no way for the customer to enter a code there. A code can only be pre-applied server-side via discount_codes when the session is created, and a confirmed session's amount is fixed at creation.

Requested behaviour

Support an "Apply discount" input on the inline/confirmed payment form, so a customer who is already on the payment step can enter a promo code, see the total update, and pay the discounted amount without leaving the merchant's page.

Either of these would solve it:

• Honour allow_discount_code on confirmed/inline sessions and render the native discount input.
• Or expose an SDK method to apply/remove a discount code on an open inline checkout (the checkout.breakdown event can already report discount, but there is no way to set one).

Why it matters

• Merchants embedding the payment form keep customers on their own page; today the only path is to build a custom promo field and re-create the session after the code is applied, which adds an extra round trip and forces the session to be recreated.
• Entering a promo code at the payment step is a familiar pattern on other payment providers, so its absence stands out during migrations.
• Customers who go looking for a code and find nowhere to enter it are a common drop-off point at the final step of checkout.