AsyncAutoResetEvent enhancement (#35593)

- Use Task.CompltedTask
- Let class sealed

sdk/openai/Azure.AI.OpenAI/src/Helpers/AsyncAutoResetEvent.cs

@@ -8,9 +8,8 @@
 namespace Azure.AI.OpenAI
 {
     // Adapted from https://devblogs.microsoft.com/pfxteam/building-async-coordination-primitives-part-2-asyncautoresetevent/
-    internal class AsyncAutoResetEvent
+    internal sealed class AsyncAutoResetEvent
     {
-        private static readonly Task s_completed = Task.FromResult(true);
         private readonly Queue<TaskCompletionSource<bool>> _waits = new Queue<TaskCompletionSource<bool>>();
         private bool _signaled;
 
@@ -21,7 +20,7 @@ public Task WaitAsync(CancellationToken cancellationToken = default)
                 if (_signaled)
                 {
                     _signaled = false;
-                    return s_completed;
+                    return Task.CompletedTask;
                 }
                 else
                 {