test add vouchers and shared service

cmd/cart/mutation_add_voucher.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+	"fmt"
+
+	"git.tornberg.me/go-cart-actor/pkg/messages"
+)
+
+func (ctx *MutationContext) AddVoucher(g *CartGrain, m *messages.AddVoucher) error {
+	if m == nil {
+		return fmt.Errorf("AddVoucher: nil payload")
+	}
+
+	voucher, err := ctx.VoucherService.GetVoucher(m.Code)
+	if err != nil {
+		return fmt.Errorf("AddVoucher: %w", err)
+	}
+
+	if voucher == nil {
+		return nil
+	}
+
+	g.Vouchers = append(g.Vouchers, voucher)
+	g.UpdateTotals()
+	return nil
+}