[browser][MT] fix .Wait in CompleteTask (#101078)

Author: pavelsavara

src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Interop/JavaScriptExports.cs

@@ -203,9 +203,11 @@ public static void CompleteTask(JSMarshalerArgument* arguments_buffer)
                     }
                 }
 
-                // this is always running on I/O thread, so it will not throw PNSE
                 // it's also OK to block here, because we know we will only block shortly, as this is just race with the other thread.
-                holder.CallbackReady?.Wait();
+                if (holder.CallbackReady != null)
+                {
+                    Thread.ForceBlockingWait(static (b) => ((ManualResetEventSlim)b!).Wait(), holder.CallbackReady);
+                }
 
                 lock (ctx)
                 {

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.WebSocket.cs

@@ -39,7 +39,6 @@ await executor.Execute(async () =>
 
 
         [Theory, MemberData(nameof(GetTargetThreads2x))]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/100931")]
         public async Task WebSocketClient_ResponseCloseInDifferentThread(Executor executor1, Executor executor2)
         {
             using var cts = CreateTestCaseTimeoutSource();