fix(nunit): use object locks so MTP compiles on net8

System.Threading.Lock is net9+; Polyfill covers net48 but not net8.0-windows, which blocked Revit 2025 sample runs.

Author: trgiangv

source/DevTools.NUnit.Core/Client/ProcessRunnerClient.cs

@@ -19,7 +19,7 @@ internal sealed class ProcessRunnerClient : IRunnerTransport, IDisposable
 
     private readonly string _runnerPath;
     private Process? _activeProcess;
-    private readonly Lock _processLock = new();
+    private readonly object _processLock = new();
 
     internal ProcessRunnerClient(string runnerPath)
     {

source/DevTools.NUnit.TestAdapter/DevToolsNUnitExecutor.cs

@@ -12,7 +12,7 @@ namespace DevTools.NUnit.TestAdapter;
 public sealed class DevToolsNUnitExecutor : ITestExecutor
 {
     private ProcessRunnerClient? _client;
-    private readonly Lock _clientLock = new();
+    private readonly object _clientLock = new();
 
     public void RunTests(IEnumerable<TestCase>? tests, IRunContext? runContext, IFrameworkHandle? frameworkHandle)
     {