[tests] Clear ResolvedCodeAnalysisRuleSet to fix CoreCompile incrementality (#10900)

CI Guardian SDL analysis (MergeGuardianDotnetAnalyzersRuleSets) regenerates a
merged .ruleset file on every build with a new timestamp. Since
 is in CoreCompile's Inputs, this causes
CoreCompile to re-run even when no source files changed, cascading through
ILLink -> _GenerateJavaStubs -> _Sign and breaking incrementality tests:
- BasicApplicationRepetitiveReleaseBuild
- CheckTimestamps
- GenerateJavaStubsAndAssembly

* Move _ClearResolvedCodeAnalysisRuleSet to XamarinAndroidProject base class

Unify the Directory.Build.targets logic so the _ClearResolvedCodeAnalysisRuleSet
target is defined once in XamarinAndroidProject (base class) rather than
duplicated in each derived project type. This fixes the
BindingLibraryIncremental test which uses XamarinAndroidBindingProject
(inherits directly from XamarinAndroidProject, not XamarinAndroidApplicationProject).

Added ExtraDirectoryBuildTargetsContent virtual property so
XamarinAndroidApplicationProject can still inject VectorDrawable workaround.

Also updated copilot-instructions.md to prefer raw string literals.

Author: jonathanpeppers

.github/copilot-instructions.md

@@ -142,6 +142,7 @@ C# code uses tabs (not spaces) and Mono style (`.editorconfig`):
 - Preserve existing formatting and comments
 - Space before `(` and `[`: `Foo ()`, `array [0]`
 - Use `""` not `string.Empty`, `[]` not `Array.Empty<T>()`
+- Prefer C# raw string literals (`"""`) for multi-line strings instead of `@""` with escaped quotes
 - Minimal diffs - don't leave random empty lines
 - Do NOT use `#region` or `#endregion`
 

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/XamarinAndroidApplicationProject.cs

@@ -29,6 +29,13 @@ namespace Xamarin.ProjectTools
 	/// <seealso cref="XamarinAndroidBindingProject"/>
 	public class XamarinAndroidApplicationProject : XamarinAndroidCommonProject
 	{
+		// Workaround for AndroidX, see: https://github.com/xamarin/AndroidSupportComponents/pull/239
+		protected override string ExtraDirectoryBuildTargetsContent => """
+			<PropertyGroup>
+				<VectorDrawableCheckBuildToolsVersionTaskBeforeTargets />
+			</PropertyGroup>
+		""";
+
 		const string default_strings_xml = @"<?xml version=""1.0"" encoding=""utf-8""?>
 <resources>
 	<string name=""hello"">Hello World, Click Me!</string>
@@ -74,16 +81,6 @@ public XamarinAndroidApplicationProject (string debugConfigurationName = "Debug"
 			SetProperty ("_FastDeploymentDiagnosticLogging", "True");
 			SupportedOSPlatformVersion = "21.0";
 
-			// Workaround for AndroidX, see: https://github.com/xamarin/AndroidSupportComponents/pull/239
-			Imports.Add (new Import (() => "Directory.Build.targets") {
-				TextContent = () =>
-					@"<Project>
-						<PropertyGroup>
-							<VectorDrawableCheckBuildToolsVersionTaskBeforeTargets />
-						</PropertyGroup>
-					</Project>"
-			});
-
 			AndroidManifest = default_android_manifest;
 			LayoutMain = default_layout_main;
 			StringsXml = default_strings_xml;

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/XamarinAndroidProject.cs

@@ -22,6 +22,13 @@ namespace Xamarin.ProjectTools
 	/// <seealso cref="XamarinAndroidProjectLanguage"/>
 	public abstract class XamarinAndroidProject : DotNetXamarinProject
 	{
+		/// <summary>
+		/// Override to provide additional MSBuild XML content inside the auto-generated Directory.Build.targets file.
+		/// The returned string is inserted as-is inside the root &lt;Project&gt; element, before the
+		/// <c>_ClearResolvedCodeAnalysisRuleSet</c> target.
+		/// </summary>
+		protected virtual string ExtraDirectoryBuildTargetsContent => "";
+
 		/// <summary>
 		/// Initializes a new instance of the XamarinAndroidProject class with the specified configuration names.
 		/// Sets the default language to C# and output type to Library.
@@ -35,6 +42,24 @@ protected XamarinAndroidProject (string debugConfigurationName = "Debug", string
 		{
 			Language = XamarinAndroidProjectLanguage.CSharp;
 			SetProperty (KnownProperties.OutputType, "Library");
+
+			// Prevent CI Guardian SDL analysis from breaking CoreCompile incrementality.
+			// Guardian's MergeGuardianDotnetAnalyzersRuleSets target regenerates a merged
+			// ruleset file on every build (new timestamp), which is in CoreCompile's Inputs
+			// via $(ResolvedCodeAnalysisRuleSet). Clearing this property ensures CoreCompile
+			// is properly incremental when source files haven't changed.
+			Imports.Add (new Import (() => "Directory.Build.targets") {
+				TextContent = () => $"""
+					<Project>
+						{ExtraDirectoryBuildTargetsContent}
+						<Target Name="_ClearResolvedCodeAnalysisRuleSet" BeforeTargets="CoreCompile" AfterTargets="ResolveCodeAnalysisRuleSet;MergeGuardianDotnetAnalyzersRuleSets">
+							<PropertyGroup>
+								<ResolvedCodeAnalysisRuleSet />
+							</PropertyGroup>
+						</Target>
+					</Project>
+				""",
+			});
 		}
 
 		/// <summary>