Issue #3609199: Add kessai_worldline kernel tests for the webhook and finalizer
Adds two kernel tests for kessai_worldline (which had none):
WorldlineWebhookTestcovers the webhook controller's verification guards (unconfigured endpoint acknowledges with 200; a bad signature is rejected with 400).WorldlineFinalizerTestcovers the finalizer's amount/currency verification (a mismatched amount or currency fails the payment; an exact match settles it), building the provider payment by hand so neither needs a live API.
Together they cover both halves of a Worldline settlement: verify the callback, then settle or reject the pending checkout payment.
Closes #3609199.