Apply NumberFormat changes to NumericUpDown (#20248)

* Add NumericUpDown NumberFormat failing test

* Force text update when NumberFormat has changed

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

src/Avalonia.Controls/NumericUpDown/NumericUpDown.cs

@@ -468,7 +468,7 @@ namespace Avalonia.Controls
         {
             if (IsInitialized)
             {
-                SyncTextAndValueProperties(false, null);
+                SyncTextAndValueProperties(false, null, true);
             }
         }
 

tests/Avalonia.Controls.UnitTests/NumericUpDownTests.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Controls.Templates;
@@ -80,6 +81,26 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Fact]
+        public void NumberFormat_Is_Applied_Immediately()
+        {
+            RunTest((control, textbox) =>
+            {
+                const decimal value = 10.11m;
+                var initialNumberFormat = new NumberFormatInfo { NumberDecimalSeparator = "." };
+                var newNumberFormat = new NumberFormatInfo { NumberDecimalSeparator = ";" };
+
+                // Establish and verify initial conditions.
+                control.NumberFormat = initialNumberFormat;
+                control.Value = value;
+                Assert.Equal(value.ToString(initialNumberFormat), control.Text);
+
+                // Check that NumberFormat is applied.
+                control.NumberFormat = newNumberFormat;
+                Assert.Equal(value.ToString(newNumberFormat), control.Text);
+            });
+        }
+
         public static IEnumerable<object[]> Increment_Decrement_TestData()
         {
             // if min and max are not defined and value was null, 0 should be ne new value after spin