Fix VerticalContentAlignment bindings and integration tests

- Bind internal grids to VerticalContentAlignment in Fluent and Simple
  themes for DatePicker and TimePicker controls.
- Add comprehensive integration tests using mocked NameScopes in
  DatePickerTests and TimePickerTests to verify visual propagation.
- Validate visual layout correctness using the ControlCatalog app.

Closes AvaloniaUI#21211

src/Avalonia.Themes.Fluent/Controls/DatePicker.xaml

@@ -98,7 +98,7 @@
                     HorizontalAlignment="Stretch"
                     VerticalAlignment="Stretch"
                     TemplatedControl.IsTemplateFocusTarget="True">
-              <Grid Name="PART_ButtonContentGrid" ColumnDefinitions="78*,Auto,132*,Auto,78*">
+              <Grid Name="PART_ButtonContentGrid" ColumnDefinitions="78*,Auto,132*,Auto,78*" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <TextBlock Name="PART_DayTextBlock" Text="{DynamicResource StringDatePickerDayText}" HorizontalAlignment="Center"
                            Padding="{DynamicResource DatePickerHostPadding}" />
                 <TextBlock Name="PART_MonthTextBlock" Text="{DynamicResource StringDatePickerMonthText}" TextAlignment="Left"

src/Avalonia.Themes.Fluent/Controls/TimePicker.xaml

@@ -97,7 +97,7 @@
                     HorizontalAlignment="Stretch"
                     VerticalAlignment="Stretch">
 
-              <Grid Name="PART_FlyoutButtonContentGrid">
+              <Grid Name="PART_FlyoutButtonContentGrid" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <!--Ignore col defs here, set in code-->
                 <Border x:Name="PART_FirstPickerHost"
                         Grid.Column="0"

src/Avalonia.Themes.Simple/Controls/DatePicker.xaml

@@ -106,7 +106,8 @@
                     TemplatedControl.IsTemplateFocusTarget="True"
                     Theme="{StaticResource SimpleDatePickerFlyoutButton}">
               <Grid Name="PART_ButtonContentGrid"
-                    ColumnDefinitions="78*,Auto,132*,Auto,78*">
+                    ColumnDefinitions="78*,Auto,132*,Auto,78*"
+                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <TextBlock Name="PART_DayTextBlock"
                            Padding="{DynamicResource DatePickerHostPadding}"
                            HorizontalAlignment="Center"

src/Avalonia.Themes.Simple/Controls/TimePicker.xaml

@@ -103,7 +103,7 @@
                     IsEnabled="{TemplateBinding IsEnabled}"
                     Theme="{StaticResource SimpleTimePickerFlyoutButton}">
 
-              <Grid Name="PART_FlyoutButtonContentGrid">
+              <Grid Name="PART_FlyoutButtonContentGrid" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <!--  Ignore col defs here, set in code  -->
                 <Border x:Name="PART_FirstPickerHost"
                         Grid.Column="0"

tests/Avalonia.Controls.UnitTests/DatePickerTests.cs

@@ -13,6 +13,7 @@ using Avalonia.Platform;
 using Avalonia.Threading;
 using Avalonia.UnitTests;
 using Avalonia.VisualTree;
+using Avalonia.Layout;
 using Moq;
 using Xunit;
 
@@ -457,5 +458,57 @@ namespace Avalonia.Controls.UnitTests
                 return contentPanel;
             });
         }
+        
+        [Fact]
+        public void VerticalContentAlignment_Should_Propagate_To_Internal_Grid()
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var picker = new DatePicker { VerticalContentAlignment = VerticalAlignment.Bottom };
+
+                picker.Template = new Avalonia.Controls.Templates.FuncControlTemplate<DatePicker>((parent, scope) =>
+                {
+                    var grid = new Grid { Name = "PART_ButtonContentGrid" };
+                    
+                    grid.Bind(Grid.VerticalAlignmentProperty, new Avalonia.Data.Binding("VerticalContentAlignment") 
+                    { 
+                        RelativeSource = new Avalonia.Data.RelativeSource(Avalonia.Data.RelativeSourceMode.TemplatedParent) 
+                    });
+
+                    var dayText = new TextBlock { Name = "PART_DayTextBlock" };
+                    var monthText = new TextBlock { Name = "PART_MonthTextBlock" };
+                    var yearText = new TextBlock { Name = "PART_YearTextBlock" };
+                    
+                    var firstSpacer = new Avalonia.Controls.Shapes.Rectangle { Name = "PART_FirstSpacer" };
+                    var secondSpacer = new Avalonia.Controls.Shapes.Rectangle { Name = "PART_SecondSpacer" };
+
+                    scope.Register("PART_ButtonContentGrid", grid);
+                    scope.Register("PART_DayTextBlock", dayText);
+                    scope.Register("PART_MonthTextBlock", monthText);
+                    scope.Register("PART_YearTextBlock", yearText);
+                    scope.Register("PART_FirstSpacer", firstSpacer);
+                    scope.Register("PART_SecondSpacer", secondSpacer);
+
+                    grid.Children.Add(dayText);
+                    grid.Children.Add(monthText);
+                    grid.Children.Add(yearText);
+                    grid.Children.Add(firstSpacer);
+                    grid.Children.Add(secondSpacer);
+
+                    return grid;
+                });
+
+                var root = new Avalonia.UnitTests.TestRoot { Child = picker };
+                root.Measure(new Avalonia.Size(100, 100));
+                root.Arrange(new Avalonia.Rect(0, 0, 100, 100));
+
+                var internalGrid = picker.GetVisualDescendants()
+                                        .OfType<Grid>()
+                                        .FirstOrDefault(g => g.Name == "PART_ButtonContentGrid");
+
+                Assert.NotNull(internalGrid);
+                Assert.Equal(VerticalAlignment.Bottom, internalGrid.VerticalAlignment);
+            }
+        }
     }
 }

tests/Avalonia.Controls.UnitTests/TimePickerTests.cs

@@ -11,6 +11,7 @@ using Avalonia.Platform;
 using Avalonia.Threading;
 using Avalonia.UnitTests;
 using Avalonia.VisualTree;
+using Avalonia.Layout;
 using Moq;
 using Xunit;
 
@@ -484,5 +485,36 @@ namespace Avalonia.Controls.UnitTests
             var timePicker = new TimePicker();
             Assert.Equal(Avalonia.Layout.VerticalAlignment.Stretch, timePicker.VerticalContentAlignment);
         }
+
+        [Fact]
+        public void VerticalContentAlignment_Should_Propagate_To_Internal_Grid()
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var picker = new TimePicker { VerticalContentAlignment = VerticalAlignment.Bottom };
+
+                picker.Template = new Avalonia.Controls.Templates.FuncControlTemplate<TimePicker>((parent, scope) =>
+                {
+                    var grid = new Grid { Name = "PART_FlyoutButtonContentGrid" };
+                    
+                    grid.Bind(Grid.VerticalAlignmentProperty, new Avalonia.Data.Binding("VerticalContentAlignment") 
+                    { 
+                        RelativeSource = new Avalonia.Data.RelativeSource(Avalonia.Data.RelativeSourceMode.TemplatedParent) 
+                    });
+                    return grid;
+                });
+
+                var root = new Avalonia.UnitTests.TestRoot { Child = picker };
+                root.Measure(new Avalonia.Size(100, 100));
+                root.Arrange(new Avalonia.Rect(0, 0, 100, 100));
+
+                var internalGrid = picker.GetVisualDescendants()
+                                        .OfType<Grid>()
+                                        .FirstOrDefault(g => g.Name == "PART_FlyoutButtonContentGrid");
+
+                Assert.NotNull(internalGrid);
+                Assert.Equal(VerticalAlignment.Bottom, internalGrid.VerticalAlignment);
+            }
+        }
     }
 }