Open API coupon validation

Pelcro today returns a generic invalid result for coupon validation across many distinct business rules (expired, wrong plan, segment, redemption limits, etc.), with inconsistent HTTP statuses and free-text messages whose location varies by case. To support our checkout flow, we need a stable, enumerated failure_reason_code on every coupon validation failure — order preview, subscription commit, and campaign-attached coupons (coupon.valid: false) — returned with one documented HTTP status at a fixed body location, identifying the coupon as the failing input, plus structured failure_reason_detail for support and ops. We will map codes to subscriber-facing copy and will manage any abuse/enumeration messaging on its side — please do not collapse distinct failure reasons in the API. Please provide a published reason-code dictionary, sample error payloads for each code, and confirmation that sticky re-validation on plan change returns plan-eligibility failures distinctly from expired/unknown codes.

Attached is a more complete requirements document. Please review.