Fix TestIndexWriterOnJRECrash fork on ARM64 hosts

GetTargetPlatform() returned "x64" on any 64-bit process, which on Apple
Silicon (and other ARM64 hosts) caused the forked vstest to abort with
"Could not find 'dotnet' host for the 'X64' architecture". The fork
never connected back to the parent's TCP listener, so the parent blocked
forever in WaitForProcessId until the assembly Timeout fired.

Detect the architecture via RuntimeInformation.ProcessArchitecture so
the fork runs in the same architecture as the parent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: paulirwin

src/Lucene.Net.Tests/Index/TestIndexWriterOnJRECrash.cs

@@ -11,6 +11,7 @@
 using System.Net;
 using System.Net.Sockets;
 using System.Reflection;
+using System.Runtime.InteropServices;
 using System.Threading;
 using BaseDirectoryWrapper = Lucene.Net.Store.BaseDirectoryWrapper;
 using Assert = Lucene.Net.TestFramework.Assert;
@@ -242,7 +243,19 @@ private string GetTargetFramework()
 
         private static string GetTargetPlatform()
         {
-            return Environment.Is64BitProcess ? "x64" : "x86";
+            // LUCENENET: Match the host process architecture so the forked vstest can
+            // locate a compatible dotnet host. On Apple Silicon and other ARM64 hosts,
+            // returning "x64" here causes the fork to fail with "Could not find 'dotnet'
+            // host for the 'X64' architecture", which leaves the parent blocked forever
+            // in WaitForProcessId.
+            return RuntimeInformation.ProcessArchitecture switch
+            {
+                Architecture.X86 => "x86",
+                Architecture.X64 => "x64",
+                Architecture.Arm => "ARM",
+                Architecture.Arm64 => "ARM64",
+                _ => Environment.Is64BitProcess ? "x64" : "x86",
+            };
         }
         #endregion