[X] Change the default value for xaml-comp (#19390)

The default value for the xaml-comp processing instruction used to be
false, meaning the a XAML ResourceDictionary without code-behind wasn't
compiled by XamlC unless the `<?xaml-comp compile="true" ?>` processing
instruction was specified.

From now on, we will compile all XAML-only RD, unless the 
`<?xaml-comp compile="false" ?>` is set.

Thsi will help with performance and trimming

src/Controls/src/SourceGen/CodeBehindGenerator.cs

@@ -295,7 +295,7 @@ static bool TryParseXaml(SourceText text, string uid, Compilation compilation, A
 
 			if (rootClass != null)
 				XmlnsHelper.ParseXmlns(rootClass.Value, out rootType, out rootClrNamespace, out var rootAsm, out var targetPlatform);
-			else if (hasXamlCompilationProcessingInstruction)
+			else if (hasXamlCompilationProcessingInstruction && root.NamespaceURI == XamlParser.MauiUri)
 			{
 				rootClrNamespace = "__XamlGeneratedCode__";
 				rootType = $"__Type{uid}";
@@ -325,13 +325,13 @@ static bool GetXamlCompilationProcessingInstruction(XmlDocument xmlDoc)
 		{
 			var instruction = xmlDoc.SelectSingleNode("processing-instruction('xaml-comp')") as XmlProcessingInstruction;
 			if (instruction == null)
-				return false;
+				return true;
 
 			var parts = instruction.Data.Split(' ', '=');
 			var indexOfCompile = Array.IndexOf(parts, "compile");
 			if (indexOfCompile != -1)
-				return parts[indexOfCompile + 1].Trim('"', '\'').Equals("true", StringComparison.OrdinalIgnoreCase);
-			return false;
+				return !parts[indexOfCompile + 1].Trim('"', '\'').Equals("false", StringComparison.OrdinalIgnoreCase);
+			return true;
 		}
 
 		static IEnumerable<(string name, string type, string accessModifier)> GetNamedFields(XmlNode root, XmlNamespaceManager nsmgr, Compilation compilation, AssemblyCaches caches, CancellationToken cancellationToken)

src/Controls/tests/Xaml.UnitTests/CecilExtensionsTests.cs

@@ -72,27 +72,28 @@ EmbeddedResource GetResource(string name)
 			"IsCompiledDefault",
 			"X2006Namespace",
 			"X2009Primitives",
+			"Validation.MissingXClass",
 		};
 
 		[Test, TestCaseSource(nameof(IsXamlTrueSource))]
 		public void IsXamlTrue(string name)
 		{
 			var resource = GetResource(name);
 			Assert.IsTrue(resource.IsXaml(new XamlCache(), assembly.MainModule, out string className), $"IsXaml should return true for '{name}'.");
-			Assert.AreEqual(className, $"{testNamespace}.{name}"); // Test cases x:Class matches the file name
+			if (!className.StartsWith("__XamlGeneratedCode__"))
+				Assert.AreEqual(className, $"{testNamespace}.{name}"); // Test cases x:Class matches the file name
 		}
 
 		static string[] IsXamlFalseSource = new[]
 		{
-			"Validation.MissingXClass",
 			"Validation.NotXaml",
-		};
-
+		};		
+	
 		[Test, TestCaseSource(nameof(IsXamlFalseSource))]
 		public void IsXamlFalse(string name)
 		{
 			var resource = GetResource(name);
 			Assert.IsFalse(resource.IsXaml(new XamlCache(), assembly.MainModule, out _), $"IsXaml should return false for '{name}'.");
-		}
+		}		
 	}
 }

src/Controls/tests/Xaml.UnitTests/Issues/Gh13209.xaml.cs

@@ -18,6 +18,8 @@ public Gh13209(bool useCompiledXaml)
 		[TestFixture]
 		class Tests
 		{
+			[TearDown] public void TearDown() => ResourceDictionary.ClearCache();
+
 			[TestCase(true), TestCase(false)]
 			public void RdWithSource(bool useCompiledXaml)
 			{

src/Controls/tests/Xaml.UnitTests/ResourceDictionaryWithSource.xaml.cs

@@ -2,6 +2,7 @@
 using Microsoft.Maui.Controls.Core.UnitTests;
 using Microsoft.Maui.Graphics;
 using NUnit.Framework;
+using NUnit.Framework.Constraints;
 
 namespace Microsoft.Maui.Controls.Xaml.UnitTests
 {
@@ -48,7 +49,7 @@ public void XRIDIsGeneratedForRDWithoutCodeBehind()
 				var resourceId = XamlResourceIdAttribute.GetResourceIdForPath(asm, "AppResources/Colors.xaml");
 				Assert.That(resourceId, Is.Not.Null);
 				var type = XamlResourceIdAttribute.GetTypeForResourceId(asm, resourceId);
-				Assert.That(type, Is.Null);
+				Assert.That(type.Name, Does.StartWith("__Type")); //xaml-comp defult to true, so this is an autogenerated code behind
 			}
 
 			[Test]