[xabt] Make static fields readonly to improve MSBuild safety (#10359)

Fixes: #5996

This PR addresses static field usage in Xamarin.Android.Build.Tasks to
improve MSBuild safety and reduce potential thread safety issues, as
identified in the audit request.

## Problem

The original issue identified 27 mutable static fields that could be
problematic in MSBuild context where tasks might run in parallel or be
cached inappropriately. Mutable static fields can cause:

1. Race conditions during parallel task execution
2. Unexpected state mutations between builds
3. Thread safety issues in IDE scenarios where SDK paths might change

## Solution

After systematically auditing all static fields in the Build.Tasks
assembly, I identified and fixed 13 static fields that should be
immutable after initialization by adding the `readonly` modifier:

**XML and Configuration Fields:**

- `ManifestDocument.AndroidXmlNamespace` & `AndroidXmlToolsNamespace` - XML namespace constants
- `ManifestDocument.androidNs` & `androidToolsNs` - Cached namespace references
- `JavaObjectsXmlFile.settings` - XML writer settings
- `SupportsGLTextureAttribute.mapping` - Manifest mapping configuration

**Regex and Parsing Fields:**

- `Aapt2Link.exraArgSplitRegEx` - Compiled regex for argument parsing  
- `ResourceIdentifier.validIdentifier` - Compiled regex for validation
- `CheckForInvalidResourceFileNames.javaKeywords` - Java keyword array
- `RtxtParser.knownTypes` - Set of known resource types

**Utility Fields:**

- `ManagedResourceParser.compareTuple` - Comparer instance
- `RemoveUnknownFiles.IsWindows` & `PathUtil.IsWindows` - Platform detection flags

## Analysis of Remaining Static Fields

The remaining static fields were analyzed and found to be appropriate:

- `MonoAndroidHelper.SupportedVersions` & `AndroidSdk` - Set once per
  build by ResolveSdksTask following established MSBuild patterns
- `AndroidLinkConfiguration.configurations` - Uses
  `ConditionalWeakTable` which is already thread-safe
- `MonoAndroidHelper.uname` - Uses `Lazy<T>` with thread-safe
  initialization

Co-authored-by: Jonathan Peppers <[email protected]>

Author: Copilot

src/Xamarin.Android.Build.Tasks/Mono.Android/SupportsGLTextureAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.App
 {
 	partial class SupportsGLTextureAttribute
 	{
-		static ManifestDocumentElement<SupportsGLTextureAttribute> mapping = new ManifestDocumentElement<SupportsGLTextureAttribute> ("supports-gl-texture") {
+		static readonly ManifestDocumentElement<SupportsGLTextureAttribute> mapping = new ManifestDocumentElement<SupportsGLTextureAttribute> ("supports-gl-texture") {
 			{
 			  "Name",
 			  "name",

src/Xamarin.Android.Build.Tasks/Tasks/Aapt2Link.cs

@@ -20,7 +20,7 @@ namespace Xamarin.Android.Tasks {
 
 	//aapt2 link -o resources.apk.bk --manifest Foo.xml --java . --custom-package com.infinitespace_studios.blankforms -R foo2 -v --auto-add-overlay
 	public class Aapt2Link : Aapt2 {
-		static Regex exraArgSplitRegEx = new Regex (@"[\""].+?[\""]|[\''].+?[\'']|[^ ]+", RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.Multiline);
+		static readonly Regex exraArgSplitRegEx = new Regex (@"[\""].+?[\""]|[\''].+?[\'']|[^ ]+", RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.Multiline);
 		public override string TaskPrefix => "A2L";
 
 		[Required]

src/Xamarin.Android.Build.Tasks/Tasks/CheckForInvalidResourceFileNames.cs

@@ -24,7 +24,7 @@ public class CheckForInvalidResourceFileNames : AndroidTask {
 		Regex fileNameWithHyphenCheck = new Regex ("[^a-zA-Z0-9_.-]+", RegexOptions.Compiled);
 
 		// Source https://docs.oracle.com/javase/tutorial/java/nutsandbolts/_keywords.html
-		static string [] javaKeywords = {
+		static readonly string [] javaKeywords = {
 			"abstract",
 			"assert",
 			"boolean",

src/Xamarin.Android.Build.Tasks/Tasks/RemoveUnknownFiles.cs

@@ -14,7 +14,7 @@ public class RemoveUnknownFiles : AndroidTask
 	{
 		public override string TaskPrefix => "RUF";
 
-		static bool IsWindows = Path.DirectorySeparatorChar == '\\';
+		static readonly bool IsWindows = Path.DirectorySeparatorChar == '\\';
 
 		[Required]
 		public ITaskItem[] Files { get; set; } = [];

src/Xamarin.Android.Build.Tasks/Utilities/JavaObjectsXmlFile.cs

@@ -14,7 +14,7 @@ namespace Xamarin.Android.Tasks;
 
 class JavaObjectsXmlFile
 {
-	static XmlWriterSettings settings = new XmlWriterSettings {
+	static readonly XmlWriterSettings settings = new XmlWriterSettings {
 		Indent = true,
 		NewLineOnAttributes = false,
 		OmitXmlDeclaration = true,

src/Xamarin.Android.Build.Tasks/Utilities/ManagedResourceParser.cs

@@ -35,7 +35,7 @@ public int Compare((int Key, CodeMemberField Value) x, (int Key, CodeMemberField
 		List<CodeTypeDeclaration> typeIds = new List<CodeTypeDeclaration> ();
 		Dictionary<CodeMemberField, CodeMemberField []> arrayMapping = new Dictionary<CodeMemberField, CodeMemberField []> ();
 		const string itemPackageId = "0x7f";
-		static CompareTuple compareTuple = new CompareTuple ();
+		static readonly CompareTuple compareTuple = new CompareTuple ();
 
 		XDocument publicXml;
 

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocument.cs

@@ -31,13 +31,13 @@ namespace Xamarin.Android.Tasks {
 
 	internal class ManifestDocument
 	{
-		public static XNamespace AndroidXmlNamespace = "http://schemas.android.com/apk/res/android";
-		public static XNamespace AndroidXmlToolsNamespace = "http://schemas.android.com/tools";
+		public static readonly XNamespace AndroidXmlNamespace = "http://schemas.android.com/apk/res/android";
+		public static readonly XNamespace AndroidXmlToolsNamespace = "http://schemas.android.com/tools";
 
 		const int maxVersionCode = 2100000000;
 
-		static XNamespace androidNs = AndroidXmlNamespace;
-		static XNamespace androidToolsNs = AndroidXmlToolsNamespace;
+		static readonly XNamespace androidNs = AndroidXmlNamespace;
+		static readonly XNamespace androidToolsNs = AndroidXmlToolsNamespace;
 		static readonly XName versionCodeAttributeName = androidNs + "versionCode";
 
 		XDocument doc;

src/Xamarin.Android.Build.Tasks/Utilities/PathUtil.cs

@@ -15,7 +15,7 @@ public static class PathUtil
 
 		internal const char VolumeSeparatorChar = ':';
 
-		static bool IsWindows = Path.DirectorySeparatorChar == '\\';
+		static readonly bool IsWindows = Path.DirectorySeparatorChar == '\\';
 		// Adapted from CoreFX sources
 		public static string GetRelativePath(string relativeTo, string path, StringComparison comparisonType = StringComparison.OrdinalIgnoreCase)
 		{

src/Xamarin.Android.Build.Tasks/Utilities/ResourceIdentifier.cs

@@ -32,7 +32,7 @@ public class ResourceIdentifier {
 		private const string Identifier = IdentifierStartCharacter + "(" + IdentifierPartCharacter + ")";
 
 		// We use [^ ...] to detect any character that is NOT a match.
-		static Regex validIdentifier = new Regex ($"[^{Identifier}]", RegexOptions.Compiled);
+		static readonly Regex validIdentifier = new Regex ($"[^{Identifier}]", RegexOptions.Compiled);
 
 		public static string CreateValidIdentifier (string identifier)
 		{

src/Xamarin.Android.Build.Tasks/Utilities/RtxtParser.cs

@@ -71,7 +71,7 @@ public class RtxtParser {
 		TaskLoggingHelper? log;
 		Dictionary<string, string>? map;
 
-		public static HashSet<string> knownTypes = new HashSet<string> () {
+		public static readonly HashSet<string> knownTypes = new HashSet<string> () {
 			"anim",
 			"animator",
 			"attr",