Add VerticalContentAlignment property to DatePicker and TimePicker

This change registers VerticalContentAlignmentProperty as a
StyledProperty using ContentControl's property as the owner for both
DatePicker and TimePicker controls to maintain framework consistency.

Basic xUnit v3 unit tests are introduced to validate property
round-trips (Top, Center, Bottom, Stretch) and to ensure the default
value correctly resolves to Stretch. This establishes the necessary
C# infrastructure before modifying the control templates.

Part of #21211: implment fluent theme layout

src/Avalonia.Controls/DateTimePickers/DatePicker.cs

@@ -75,6 +75,21 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<bool> YearVisibleProperty =
             AvaloniaProperty.Register<DatePicker, bool>(nameof(YearVisible), true);
 
+        /// <summary>
+        /// Defines the <see cref="VerticalContentAlignment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
+            ContentControl.VerticalContentAlignmentProperty.AddOwner<DatePicker>();
+
+        /// <summary>
+        /// Gets or sets the vertical alignment of the content within the control.
+        /// </summary>
+        public VerticalAlignment VerticalContentAlignment
+        {
+            get => GetValue(VerticalContentAlignmentProperty);
+            set => SetValue(VerticalContentAlignmentProperty, value);
+        }
+
         /// <summary>
         /// Defines the <see cref="SelectedDate"/> Property
         /// </summary>

src/Avalonia.Controls/DateTimePickers/TimePicker.cs

@@ -50,6 +50,20 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<string> ClockIdentifierProperty =
            AvaloniaProperty.Register<TimePicker, string>(nameof(ClockIdentifier), "12HourClock", coerce: CoerceClockIdentifier);
         
+        /// <summary>
+        /// Defines the <see cref="VerticalContentAlignment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
+            ContentControl.VerticalContentAlignmentProperty.AddOwner<TimePicker>();
+
+        /// <summary>
+        /// Gets or sets the vertical alignment of the content within the control.
+        /// </summary>
+        public VerticalAlignment VerticalContentAlignment
+        {
+            get => GetValue(VerticalContentAlignmentProperty);
+            set => SetValue(VerticalContentAlignmentProperty, value);
+        }
         /// <summary>
         /// Defines the <see cref="UseSeconds"/> property
         /// </summary>

tests/Avalonia.Controls.UnitTests/DatePickerTests.cs

@@ -371,6 +371,24 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Theory]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Top)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Center)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Bottom)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Stretch)]
+        public void VerticalContentAlignment_RoundTrips(Avalonia.Layout.VerticalAlignment value)
+        {
+            var datePicker = new DatePicker { VerticalContentAlignment = value };
+            Assert.Equal(value, datePicker.VerticalContentAlignment);
+        }
+
+        [Fact]
+        public void VerticalContentAlignment_Default_Is_Stretch()
+        {
+            var datePicker = new DatePicker();
+            Assert.Equal(Avalonia.Layout.VerticalAlignment.Stretch, datePicker.VerticalContentAlignment);
+        }
+
         private static IControlTemplate CreatePickerTemplate()
         {
             return new FuncControlTemplate((_, scope) =>

tests/Avalonia.Controls.UnitTests/TimePickerTests.cs

@@ -466,5 +466,23 @@ namespace Avalonia.Controls.UnitTests
                 return contentPanel;
             });
         }
+
+        [Theory]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Top)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Center)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Bottom)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Stretch)]
+        public void VerticalContentAlignment_RoundTrips(Avalonia.Layout.VerticalAlignment value)
+        {
+            var timePicker = new TimePicker { VerticalContentAlignment = value };
+            Assert.Equal(value, timePicker.VerticalContentAlignment);
+        }
+
+        [Fact]
+        public void VerticalContentAlignment_Default_Is_Stretch()
+        {
+            var timePicker = new TimePicker();
+            Assert.Equal(Avalonia.Layout.VerticalAlignment.Stretch, timePicker.VerticalContentAlignment);
+        }
     }
 }