Follow kessai's rename of a gateway instance to a payment account

Problem/Motivation

kessai #3621928 renames the config entity a payment names: the entity type id becomes kessai_payment_account, the interface becomes PaymentAccountInterface, and the collection route becomes entity.kessai_payment_account.collection. Two places here still name the old ones, and one of them is production code.

BookingPaymentActionBase::buildConfigurationForm() builds the help text under Payment gateway override with Url::fromRoute('entity.kessai_gateway_instance.collection'). Once kessai lands there is no such route, so the call raises RouteNotFoundException and the step configuration form of every booking payment action fails to render rather than showing an empty link.

PaymentGatewayResolverTest imports GatewayInstanceInterface and loads the kessai_gateway_instance storage, so it errors instead of failing, which says nothing about the resolver.

Proposed resolution

Name the route, interface and storage id kessai now has. Nothing else moves: the resolver already asks PaymentClientInterface::isGatewayAvailable(), which keeps its name, and the stored yoyaku_payment.settings:default_gateway keeps holding an account id.

Remaining tasks

This depends on kessai #3621928. The pipeline here cannot go green until that is merged and packages.drupal.org has repackaged 1.x-dev, so the merge request stays a draft until then.

User interface changes

None, beyond the help text's link now pointing at the accounts list where it moved to, /admin/payments/accounts.

API changes

None.

Data model changes

None.

AI-Generated: Yes (Claude Code was used to help draft this issue summary and to write the code on the merge request. I reviewed and ran the work myself before posting it.)