PIPRES-794 Fix credit card 422 amount error with active payment surcharge

[TLabutis]

Ticket

PIPRES-794 (GitHub #1437) - credit card payments failing with a Mollie 422 "amount is off".

Cause

On the Payments API path, PaymentMethodService::getPaymentData() passed the surcharge-inclusive total ($totalAmount) into CartLinesService::getCartLines(). The payment fee is added there as its own line, so folding it into the amount left the per-line rounding residual conflated with the fee and never compensated. The order lines then summed to a cent more or less than the order amount, which Mollie rejects with a 422. Triggered when a payment surcharge is active and PS_ROUND_TYPE rounds per line (or on the total).

Fix

Pass the pre-fee $amount into getCartLines(), matching the Orders API path. The surcharge is still added as its own line and the amount sent to Mollie is unchanged. One line (plus a comment).

Testing

• Reproduced and verified before/after through a real storefront checkout under the merchant's confirmed settings (round on each line, round half up, percentage surcharge 2.9%).
• 136-scenario regression matrix (fixed vs unpatched): only surcharge + rounding-gap cases on the Payments path change and now pass; every other scenario is byte-identical, including a no-op when no surcharge is active.
• No regression to the 6.4.3 whole-number distribution or the 6.4.4 high-value VAT fix (PIPRES-781): the per-line VAT invariant holds with 0 violations.

Note

An unrelated rounding bug (free-shipping voucher on a paid carrier) was found during this work and filed separately as PIPRES-795.