Seed: price calculator with discount tests

Mini project for agentd live testing: three of the five tests fail on main.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/Prices.Tests/PriceCalculatorTests.cs

@@ -0,0 +1,36 @@
+using Xunit;
+
+namespace Prices.Tests;
+
+public sealed class PriceCalculatorTests
+{
+    [Fact]
+    public void No_discount_keeps_the_price()
+    {
+        Assert.Equal(100m, PriceCalculator.ApplyDiscount(100m, 0m));
+    }
+
+    [Fact]
+    public void A_full_discount_makes_it_free()
+    {
+        Assert.Equal(0m, PriceCalculator.ApplyDiscount(200m, 100m));
+    }
+
+    [Fact]
+    public void A_quarter_off_200_is_150()
+    {
+        Assert.Equal(150m, PriceCalculator.ApplyDiscount(200m, 25m));
+    }
+
+    [Fact]
+    public void Half_off_80_is_40()
+    {
+        Assert.Equal(40m, PriceCalculator.ApplyDiscount(80m, 50m));
+    }
+
+    [Fact]
+    public void A_discount_above_100_percent_is_refused()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => PriceCalculator.ApplyDiscount(100m, 101m));
+    }
+}

tests/Prices.Tests/Prices.Tests.csproj

@@ -0,0 +1,20 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <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>
+    <ProjectReference Include="..\..\src\Prices\Prices.csproj" />
+  </ItemGroup>
+
+</Project>