Add round 2: split multi-step OrderBook tests (example created by participant)

- Add OrderBookDomain: in-memory order book with Pending →
  WaitingForPayment → Fulfilling → Delivered state machine, payment
  window based on passed-in timestamps, cancellation rules
- Add OrderBookTests: 7 integration tests in given/when/then/when/then
  anti-pattern style as starting material for the exercise
- Learning hour: add round 2 where participants create the example
  themselves before showing it to the agent (no refactored test given)
- Fix sln: OrderTests Debug config was mapped to Release

exercises/show-dont-tell/tests/OrderBookTests/OrderBookLifecycleTests.cs

@@ -0,0 +1,114 @@
+using NUnit.Framework;
+using OrderBookDomain;
+
+namespace OrderBookTests;
+
+public class OrderBookLifecycleTests
+{
+    private static readonly DateTimeOffset Day0 = new(2025, 6, 1, 0, 0, 0, TimeSpan.Zero);
+
+    private static DateTimeOffset At(int day) => Day0.AddDays(day);
+
+    [Test]
+    public void FinalizeThenPay_MovesOrderThroughTwoStates()
+    {
+        var book = new OrderBook();
+        var order = book.OpenOrder("anna");
+
+        book.FinalizeOrder(order.Id, At(1));
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.WaitingForPayment));
+
+        book.PayOrder(order.Id, At(2));
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.Fulfilling));
+    }
+
+    [Test]
+    public void PayThenShip_DeliversTheOrder()
+    {
+        var book = new OrderBook();
+        var order = book.OpenOrder("anna");
+
+        book.FinalizeOrder(order.Id, At(1));
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.WaitingForPayment));
+
+        book.PayOrder(order.Id, At(2));
+        book.ShipOrder(order.Id);
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.Delivered));
+    }
+
+    [Test]
+    public void PayWithinWindow_ThenLatePayOnSecondOrder_CancelsOnlyThatOrder()
+    {
+        var book = new OrderBook();
+        var first = book.OpenOrder("anna");
+        var second = book.OpenOrder("bram");
+
+        book.FinalizeOrder(first.Id, At(1));
+        book.FinalizeOrder(second.Id, At(1));
+        Assert.That(book.Orders, Has.Count.EqualTo(2));
+
+        book.PayOrder(first.Id, At(5));
+        Assert.That(book.GetOrder(first.Id).Status, Is.EqualTo(OrderStatus.Fulfilling));
+
+        Assert.Throws<InvalidOperationException>(() => book.PayOrder(second.Id, At(30)));
+        Assert.That(book.GetOrder(second.Id).Status, Is.EqualTo(OrderStatus.Cancelled));
+    }
+
+    [Test]
+    public void PayBeforeFinalize_Throws_ThenFinalizeStillWorks()
+    {
+        var book = new OrderBook();
+        var order = book.OpenOrder("anna");
+
+        Assert.Throws<InvalidOperationException>(() => book.PayOrder(order.Id, At(1)));
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.Pending));
+
+        book.FinalizeOrder(order.Id, At(2));
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.WaitingForPayment));
+    }
+
+    [Test]
+    public void CancelPendingOrder_ThenPayAfterCancel_ThrowsAgain()
+    {
+        var book = new OrderBook();
+        var order = book.OpenOrder("anna");
+
+        book.CancelOrder(order.Id);
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.Cancelled));
+
+        Assert.Throws<InvalidOperationException>(() => book.PayOrder(order.Id, At(1)));
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.Cancelled));
+    }
+
+    [Test]
+    public void ShippedOrder_CannotBeCancelled()
+    {
+        var book = new OrderBook();
+        var order = book.OpenOrder("anna");
+
+        book.FinalizeOrder(order.Id, At(1));
+        book.PayOrder(order.Id, At(2));
+        book.ShipOrder(order.Id);
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.Delivered));
+
+        Assert.Throws<InvalidOperationException>(() => book.CancelOrder(order.Id));
+        Assert.That(book.GetOrder(order.Id).Status, Is.EqualTo(OrderStatus.Delivered));
+    }
+
+    [Test]
+    public void TwoCustomers_InterleaveTransitions_Independently()
+    {
+        var book = new OrderBook();
+        var annas = book.OpenOrder("anna");
+        var brams = book.OpenOrder("bram");
+
+        book.FinalizeOrder(brams.Id, At(1));
+        Assert.That(book.GetOrder(annas.Id).Status, Is.EqualTo(OrderStatus.Pending));
+        Assert.That(book.GetOrder(brams.Id).Status, Is.EqualTo(OrderStatus.WaitingForPayment));
+
+        book.PayOrder(brams.Id, At(2));
+        book.CancelOrder(annas.Id);
+        Assert.That(book.GetOrder(annas.Id).Status, Is.EqualTo(OrderStatus.Cancelled));
+        Assert.That(book.GetOrder(brams.Id).Status, Is.EqualTo(OrderStatus.Fulfilling));
+    }
+}