Add NullableDefaultNumericConverter unit tests

StabilityMatrix.Tests/Avalonia/Converters/NullableDefaultNumericConverterTests.cs

@@ -0,0 +1,44 @@
+using System.Globalization;
+using StabilityMatrix.Avalonia.Converters;
+
+namespace StabilityMatrix.Tests.Avalonia.Converters;
+
+[TestClass]
+public class NullableDefaultNumericConverterTests
+{
+    [TestMethod]
+    public void Convert_IntToDecimal_ValueReturnsNullable()
+    {
+        const int value = 123;
+
+        var converter = NullableDefaultNumericConverters.IntToDecimal;
+
+        var result = converter.Convert(value, typeof(decimal?), null, CultureInfo.InvariantCulture);
+
+        Assert.AreEqual((decimal?)123, result);
+    }
+
+    [TestMethod]
+    public void ConvertBack_IntToDecimal_NullableReturnsDefault()
+    {
+        decimal? value = null;
+
+        var converter = NullableDefaultNumericConverters.IntToDecimal;
+
+        var result = converter.ConvertBack(value, typeof(int), null, CultureInfo.InvariantCulture);
+
+        Assert.AreEqual(0, result);
+    }
+
+    [TestMethod]
+    public void ConvertBack_IntToDouble_NanReturnsDefault()
+    {
+        const double value = double.NaN;
+
+        var converter = new NullableDefaultNumericConverter<int, double>();
+
+        var result = converter.ConvertBack(value, typeof(int), null, CultureInfo.InvariantCulture);
+
+        Assert.AreEqual(0, result);
+    }
+}