attempt to fix #98406 (#99317)

Author: pavelsavara

src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSProxyContext.cs

@@ -280,16 +280,14 @@ public nint AllocJSVHandle()
         {
             lock (this)
             {
+                ObjectDisposedException.ThrowIf(_isDisposed, this);
+
                 if (JSVHandleFreeList.Count > 0)
                 {
                     var jsvHandle = JSVHandleFreeList[JSVHandleFreeList.Count - 1];
                     JSVHandleFreeList.RemoveAt(JSVHandleFreeList.Count - 1);
                     return jsvHandle;
                 }
-                if (NextJSVHandle == IntPtr.Zero)
-                {
-                    NextJSVHandle = -2;
-                }
                 return NextJSVHandle--;
             }
         }

src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Marshaling/JSMarshalerArgument.Task.cs

@@ -189,7 +189,7 @@ internal void ToJSDynamic(Task? value)
             var taskHolder = ctx.CreateCSOwnedProxy(slot.JSHandle);
 
 #if FEATURE_WASM_MANAGED_THREADS
-            task.ContinueWith(Complete, taskHolder, TaskScheduler.FromCurrentSynchronizationContext());
+            task.ContinueWith(Complete, taskHolder, CancellationToken.None, TaskContinuationOptions.RunContinuationsAsynchronously, TaskScheduler.FromCurrentSynchronizationContext());
 #else
             task.ContinueWith(Complete, taskHolder, TaskScheduler.Current);
 #endif
@@ -273,7 +273,7 @@ public void ToJS(Task? value)
             var taskHolder = ctx.CreateCSOwnedProxy(slot.JSHandle);
 
 #if FEATURE_WASM_MANAGED_THREADS
-            task.ContinueWith(Complete, taskHolder, TaskScheduler.FromCurrentSynchronizationContext());
+            task.ContinueWith(Complete, taskHolder, CancellationToken.None, TaskContinuationOptions.RunContinuationsAsynchronously, TaskScheduler.FromCurrentSynchronizationContext());
 #else
             task.ContinueWith(Complete, taskHolder, TaskScheduler.Current);
 #endif
@@ -350,7 +350,7 @@ public void ToJS<T>(Task<T>? value, ArgumentToJSCallback<T> marshaler)
             var taskHolder = ctx.CreateCSOwnedProxy(slot.JSHandle);
 
 #if FEATURE_WASM_MANAGED_THREADS
-            task.ContinueWith(Complete, new HolderAndMarshaler<T>(taskHolder, marshaler), TaskScheduler.FromCurrentSynchronizationContext());
+            task.ContinueWith(Complete, new HolderAndMarshaler<T>(taskHolder, marshaler), CancellationToken.None, TaskContinuationOptions.RunContinuationsAsynchronously, TaskScheduler.FromCurrentSynchronizationContext());
 #else
             task.ContinueWith(Complete, new HolderAndMarshaler<T>(taskHolder, marshaler), TaskScheduler.Current);
 #endif