Pre-fill a discount code on static payment links

Problem

Static payment links accept no discount parameter. Merchants sharing a link with a coupon have to tell the customer to type the code in themselves, and customers often get it wrong or miss it entirely.

Checkout Sessions already support discount_code, so this is a gap between the two checkout methods.

Passing discount_code, discountCode, discount, coupon or promo_code on a /buy/{product_id} link all produce a session with the discount fields empty, identical to passing no parameter at all. The parameter is silently dropped.

Ask

1. Support a discount code query parameter on static payment links, applied to the session the link creates, the same way discount_code works on Checkout Sessions.
2. Ideally lock it so the customer cannot remove or replace it.

Why

Merchants share payment links directly with customers, often one to one. Making the customer type a code they were already given is an avoidable drop-off.