feat(AutoCompleteBox): Added MaxLength (#14592)

* feat(AutoCompleteBox): Added MaxLength

* fix: Address review

src/Avalonia.Controls/AutoCompleteBox/AutoCompleteBox.Properties.cs

@@ -167,6 +167,12 @@ namespace Avalonia.Controls
             AvaloniaProperty.Register<AutoCompleteBox, Func<string?, CancellationToken, Task<IEnumerable<object>>>?>(
                 nameof(AsyncPopulator));
 
+        /// <summary>
+        /// Defines the <see cref="MaxLength"/> property
+        /// </summary>
+        public static readonly StyledProperty<int> MaxLengthProperty =
+            TextBox.MaxLengthProperty.AddOwner<AutoCompleteBox>();
+
         /// <summary>
         /// Gets or sets the caret index
         /// </summary>
@@ -466,5 +472,16 @@ namespace Avalonia.Controls
             get => GetValue(ItemsSourceProperty);
             set => SetValue(ItemsSourceProperty, value);
         }
+        
+
+        /// <summary>
+        /// Gets or sets the maximum number of characters that the <see cref="AutoCompleteBox"/> can accept.
+        /// This constraint only applies for manually entered (user-inputted) text.
+        /// </summary>
+        public int MaxLength
+        {
+            get => GetValue(MaxLengthProperty);
+            set => SetValue(MaxLengthProperty, value);
+        }
     }
 }

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

@@ -44,7 +44,9 @@
                      CaretIndex="{TemplateBinding CaretIndex, Mode=TwoWay}"
                      Padding="{TemplateBinding Padding}"
                      Margin="0"
-                     DataValidationErrors.Errors="{TemplateBinding (DataValidationErrors.Errors)}" />
+                     DataValidationErrors.Errors="{TemplateBinding (DataValidationErrors.Errors)}"
+                     MaxLength="{TemplateBinding MaxLength}"
+                     />
             <Popup Name="PART_Popup" 
                    WindowManagerAddShadowHint="False"
                    MinWidth="{Binding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}" 

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

@@ -18,7 +18,9 @@
                    CornerRadius="{TemplateBinding CornerRadius}"
                    CaretIndex="{TemplateBinding CaretIndex, Mode=TwoWay}"
                    DataValidationErrors.Errors="{TemplateBinding (DataValidationErrors.Errors)}"
-                   Watermark="{TemplateBinding Watermark}" />
+                   Watermark="{TemplateBinding Watermark}"
+                   MaxLength="{TemplateBinding MaxLength}"
+                   />
 
           <Popup Name="PART_Popup"
                  MinWidth="{Binding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}"