test: Add runtime test for NumberBox NumberFormatter

src/Uno.UI.RuntimeTests/Tests/Microsoft_UI_Xaml_Controls/Given_NumberBox.cs

@@ -7,6 +7,7 @@
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Media;
 using Uno.UI.RuntimeTests.Helpers;
+using Windows.Globalization.NumberFormatting;
 using static Private.Infrastructure.TestServices;
 
 #if WINAPPSDK
@@ -54,5 +55,34 @@ public async Task When_NB_Fluent_And_Theme_Changed()
 
 		Assert.AreEqual(lightThemeForeground, (placeholderTextContentPresenter.Foreground as SolidColorBrush)?.Color);
 	}
+
+	[TestMethod]
+	public async Task NumberBox_Should_Apply_CustomFormatter()
+	{
+		var numberBox = new NumberBox();
+
+		WindowHelper.WindowContent = numberBox;
+		await WindowHelper.WaitForLoaded(numberBox);
+
+		var customFormatter = new CustomNumberFormatter();
+		numberBox.NumberFormatter = customFormatter;
+
+		numberBox.Value = 123.456;
+		var formattedText = numberBox.Text;
+
+		Assert.AreEqual("123.46 units", formattedText);
+	}
+}
+
+internal class CustomNumberFormatter : INumberFormatter2, INumberParser
+{
+	public string FormatDouble(double value) => value.ToString("0.00") + " units";
+	public double? ParseDouble(string text) => throw new NotImplementedException();
+
+	public string FormatInt(long value) => throw new NotImplementedException();
+	public string FormatUInt(ulong value) => throw new NotImplementedException();
+
+	public long? ParseInt(string text) => throw new NotImplementedException();
+	public ulong? ParseUInt(string text) => throw new NotImplementedException();
 }
 #endif