fix(07): consolidate saga contracts and add OrderSubmittedConsumer

- Remove duplicate Contracts.cs (07-02), use saga Contracts.cs (07-03)
- Add OrderSubmittedConsumer to bridge domain event to CheckoutStarted saga event
- Update SimulatePaymentCommandHandler to use saga namespace contracts

Author: baotoq

src/MicroCommerce.ApiService/Features/Ordering/Application/Commands/SimulatePayment/SimulatePaymentCommandHandler.cs

@@ -1,5 +1,6 @@
 using MassTransit;
 using MediatR;
+using MicroCommerce.ApiService.Features.Ordering.Application.Saga;
 using MicroCommerce.ApiService.Features.Ordering.Domain.ValueObjects;
 using MicroCommerce.ApiService.Features.Ordering.Infrastructure;
 using Microsoft.EntityFrameworkCore;

src/MicroCommerce.ApiService/Features/Ordering/Application/Consumers/OrderSubmittedConsumer.cs

@@ -0,0 +1,42 @@
+using MassTransit;
+using MicroCommerce.ApiService.Features.Ordering.Application.Saga;
+using MicroCommerce.ApiService.Features.Ordering.Domain.Events;
+using MicroCommerce.ApiService.Features.Ordering.Domain.ValueObjects;
+using MicroCommerce.ApiService.Features.Ordering.Infrastructure;
+using Microsoft.EntityFrameworkCore;
+
+namespace MicroCommerce.ApiService.Features.Ordering.Application.Consumers;
+
+/// <summary>
+/// Bridges the domain event (OrderSubmittedDomainEvent) to the checkout saga (CheckoutStarted).
+/// Domain events are thin (ID-only); this consumer loads order data and publishes the saga-initiating event.
+/// </summary>
+public sealed class OrderSubmittedConsumer(OrderingDbContext context)
+    : IConsumer<OrderSubmittedDomainEvent>
+{
+    public async Task Consume(ConsumeContext<OrderSubmittedDomainEvent> context1)
+    {
+        OrderId orderId = OrderId.From(context1.Message.OrderId);
+
+        Domain.Entities.Order? order = await context.Orders
+            .Include(o => o.Items)
+            .FirstOrDefaultAsync(o => o.Id == orderId);
+
+        if (order is null)
+        {
+            return;
+        }
+
+        List<CheckoutItem> items = order.Items
+            .Select(i => new CheckoutItem { ProductId = i.ProductId, Quantity = i.Quantity })
+            .ToList();
+
+        await context1.Publish(new CheckoutStarted
+        {
+            OrderId = order.Id.Value,
+            BuyerId = order.BuyerId,
+            BuyerEmail = order.BuyerEmail,
+            Items = items
+        });
+    }
+}