[Testing] Added UI Test for manual test D3

src/Controls/samples/Controls.Sample.UITests/Issues/Issue18645.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue18645">
+    <VerticalStackLayout>
+        <Label 
+            AutomationId="WaitForStubControl"
+            Text="1. Input characters in the editor until it won't accept more input." />
+        <Label 
+            Text="2. The test fails if the maximum number of characters is more or less than 5 characters." />
+        <Editor
+            AutomationId="IssueEditor"
+            MaxLength="5" />
+    </VerticalStackLayout>
+</ContentPage>

src/Controls/samples/Controls.Sample.UITests/Issues/Issue18645.xaml.cs

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+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.ManualTest, "D3", "Editor MaxLength property works as expected", PlatformAffected.All)]
+	public partial class Issue18645 : ContentPage
+	{
+		public Issue18645()
+		{
+			InitializeComponent();
+		}
+	}
+}

src/Controls/tests/UITests/Tests/Issues/Issue18645.cs

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+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues
+{
+	public class Issue18645 : _IssuesUITest
+	{
+		public Issue18645(TestDevice device) : base(device)
+		{
+		}
+
+		public override string Issue => "Editor MaxLength property works as expected";
+
+		[Test]
+		public void EditorMaxLengthWorks()
+		{
+			App.WaitForElement("WaitForStubControl");
+
+			// 1. Input characters in the editor until it won't accept more input.
+			App.EnterText("IssueEditor", "123456789");
+
+			// 2. The test fails if the maximum number of characters is more
+			// or less than 5 characters.
+			var editorText = App.FindElement("IssueEditor").GetText();
+
+			// MaxLines = 5
+			Assert.AreEqual("12345", editorText);
+		}
+	}
+}