refactor/checkout (#8)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: #8
Co-authored-by: Mats Törnberg <mats@tornberg.me>
Co-committed-by: Mats Törnberg <mats@tornberg.me>

pkg/checkout/mutation_remove_delivery.go

@@ -0,0 +1,38 @@
+package checkout
+
+import (
+	"fmt"
+
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+// mutation_remove_delivery.go
+//
+// Registers the RemoveDelivery mutation.
+//
+// Behavior:
+//   - Removes the delivery entry whose Id == payload.Id.
+//   - If not found, returns an error.
+//   - Items previously associated with that delivery simply become "without delivery";
+//     subsequent delivery mutations can reassign them.
+
+func HandleRemoveDelivery(g *CheckoutGrain, m *messages.RemoveDelivery) error {
+	if m == nil {
+		return fmt.Errorf("RemoveDelivery: nil payload")
+	}
+	targetID := uint32(m.Id)
+	index := -1
+	for i, d := range g.Deliveries {
+		if d.Id == targetID {
+			index = i
+			break
+		}
+	}
+	if index == -1 {
+		return fmt.Errorf("RemoveDelivery: delivery id %d not found", m.Id)
+	}
+
+	// Remove delivery (order not preserved beyond necessity)
+	g.Deliveries = append(g.Deliveries[:index], g.Deliveries[index+1:]...)
+	return nil
+}