Add cross-service integration tests for Order → Product → Notification flow

[devin-ai-integration]

Summary

Adds 16 cross-service integration tests exercising the full Order → Product → Notification flow, along with the minimum service implementations needed to support them.

Service implementations (previously scaffold stubs):

• Product service: ProductEntity, IProductRepository/ProductRepository, full ProductController with POST /api/product and GET /api/product/{id}, EnsureCreated in Program.cs.
• Order service: OrderEntity, IOrderRepository/OrderRepository, OrderController.Create that:
  1. Validates CustomerId != Guid.Empty and Quantity > 0
  2. Calls GET /api/product/{id} on Product service to validate product existence and fetch price
  3. Computes TotalAmount = product.Price * quantity
  4. Persists the order
  5. Publishes OrderPlacedEvent via POST /api/notification/events/order-placed to Notification service

Bug fix: All 5 service .API.csproj files referenced Shared projects with ..\..\Shared\... (resolves to Services/Shared/ — doesn't exist). Fixed to ..\..\..\Shared\....

Test infrastructure:

• docker-compose.integration.yml — postgres + rabbitmq + product/order/notification services with health checks
• DockerComposeFixture (xUnit ICollectionFixture) — starts stack, polls /healthz, tears down with docker compose down -v
• TestPriorityAttribute + PriorityOrderer for deterministic test ordering

Integration tests (OrderProductNotificationFlowTests):

1. Create product → verify 201 + correct fields
2. GET product → verify retrieval
3. Create product → place order → verify 201, productName, unitPrice, totalAmount computed from product
4. GET order → verify retrieval
5. Place order → poll GET /api/notification → verify notification has matching orderId, customerId, orderTotal, rendered subject
6. Verify notification HTML preview (GET /api/notification/{id}/preview) contains Order Confirmed
7. Order with non-existent productId → 400 + "does not exist"
8. Order price/name pulled from actual product data
   9–12. Error scenarios: empty customerId, zero/negative quantity, empty productId → 400
   13–15. GET non-existent order/product/notification → 404
9. Full E2E: 2 products, 2 orders, 2 distinct notifications with correct totals

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/0b08b35ba5664b54807c3f392a3809c1
Requested by: @bsmitches

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring