split cart and checkout and checkout vs payments

pkg/checkout/mutation_payment_declined.go

@@ -0,0 +1,28 @@
+package checkout
+
+import (
+	"errors"
+	"time"
+
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+func asPointer[T any](value T) *T {
+	return &value
+}
+
+var ErrPaymentNotFound = errors.New("payment not found")
+
+func HandlePaymentDeclined(g *CheckoutGrain, m *messages.PaymentDeclined) error {
+
+	payment, found := g.FindPayment(m.PaymentId)
+	if !found {
+		return ErrPaymentNotFound
+	}
+
+	payment.CompletedAt = asPointer(time.Now())
+	payment.Status = "failed"
+	g.PaymentInProgress--
+
+	return nil
+}