Initial commit

tests/OrderTests/OrderStateConstraint.cs

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+using System.Linq;
+using NUnit.Framework.Constraints;
+using OrderDomain;
+
+namespace OrderTests;
+
+/// <summary>
+/// Custom NUnit matcher that bundles multiple order assertions into one.
+/// Participants: you do not need to understand how this works —
+/// just recognize the pattern and use it as an example for the agent.
+/// </summary>
+public static class Has
+{
+    public static IResolveConstraint OrderState(decimal expectedTotal, decimal[] expectedLineValues)
+    {
+        return new OrderStateConstraint(expectedTotal, expectedLineValues);
+    }
+}
+
+public class OrderStateConstraint : Constraint
+{
+    private readonly decimal _expectedTotal;
+    private readonly decimal[] _expectedLineValues;
+
+    public OrderStateConstraint(decimal expectedTotal, decimal[] expectedLineValues)
+        : base($"order with total {expectedTotal} and line values [{string.Join(", ", expectedLineValues)}]")
+    {
+        _expectedTotal = expectedTotal;
+        _expectedLineValues = expectedLineValues;
+    }
+
+    public override ConstraintResult ApplyTo<TActual>(TActual actual)
+    {
+        var order = (Order)(object)actual!;
+        var actualTotal = order.Total;
+        var actualLineValues = order.Lines.Select(l => l.Value).ToArray();
+
+        var totalMatch = actualTotal == _expectedTotal;
+        var linesMatch = actualLineValues.SequenceEqual(_expectedLineValues);
+
+        var messages = new List<string>();
+        if (!totalMatch)
+            messages.Add($"Total: expected {_expectedTotal}, got {actualTotal}");
+        if (!linesMatch)
+            messages.Add($"Line values: expected [{string.Join(", ", _expectedLineValues)}], got [{string.Join(", ", actualLineValues)}]");
+
+        return new ConstraintResult(this, actual, totalMatch && linesMatch);
+    }
+}