Exercise baseline

tests/TestData/ClientBuilder.cs

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+using Domain;
+
+namespace TestData;
+
+/// <summary>Test data builder for a <see cref="Client"/> (TestDataBuilder pattern); <see cref="ValidClient"/> starts fresh — exactly what <c>new Client()</c> gives.</summary>
+public sealed class ClientBuilder
+{
+    private readonly Client _client = new();
+
+    /// <summary>A valid, fresh client — the neutral starting point for tests.</summary>
+    public static ClientBuilder ValidClient() => new();
+
+    public ClientBuilder WithId(Guid id)
+    {
+        _client.Id = id;
+        return this;
+    }
+
+    public ClientBuilder WithName(string name)
+    {
+        _client.Name = name;
+        return this;
+    }
+
+    /// <summary>Adds an order to the client (navigation fix-up wires the back-reference).</summary>
+    public ClientBuilder WithOrder(Order order)
+    {
+        _client.Orders.Add(order);
+        return this;
+    }
+
+    /// <summary>Builds the given builder and adds its order — chaining sugar for <c>.WithOrder(builder.Build())</c>.</summary>
+    public ClientBuilder WithOrder(OrderBuilder order) => WithOrder(order.Build());
+
+    public Client Build() => _client;
+}

tests/TestData/OrderBuilder.cs

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+using Domain;
+
+namespace TestData;
+
+/// <summary>Test data builder for an <see cref="Order"/> (TestDataBuilder pattern); the only place tests touch an order's construction, so tests survive domain evolution.</summary>
+public sealed class OrderBuilder
+{
+    private readonly Order _order = new();
+
+    /// <summary>A valid, fresh order — the neutral starting point for tests.</summary>
+    public static OrderBuilder ValidOrder() => new();
+
+    public OrderBuilder WithId(Guid id)
+    {
+        _order.Id = id;
+        return this;
+    }
+
+    public OrderBuilder WithDescription(string description)
+    {
+        _order.Description = description;
+        return this;
+    }
+
+    /// <summary>Sets the total in cents (<c>150_00</c> = €150.00).</summary>
+    public OrderBuilder WithTotal(int totalInCents)
+    {
+        _order.TotalInCents = totalInCents;
+        return this;
+    }
+
+    public Order Build() => _order;
+}

tests/TestData/TestData.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Domain\Domain.csproj" />
+  </ItemGroup>
+
+</Project>