Client channel: pool the state machine of SendRequestAsync

Every request allocated the async state machine box of
UaSCUaBinaryClientChannel.SendRequestAsync (~170 B). On .NET 6 and later
it uses PoolingAsyncValueTaskMethodBuilder, like the transport's send and
receive; the callers await the returned task once.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Opc.Ua.Core/Stack/Tcp/UaSCBinaryClientChannel.cs

@@ -358,6 +358,14 @@ namespace Opc.Ua.Bindings
         /// <exception cref="ArgumentNullException"><paramref name="request"/> is <c>null</c>.</exception>
         /// <exception cref="ArgumentException"></exception>
         /// <exception cref="ServiceResultException"></exception>
+        /// <remarks>
+        /// The state machine of a request is pooled, so the returned task must be awaited
+        /// once, like any value task.
+        /// </remarks>
+#if NET6_0_OR_GREATER
+        [System.Runtime.CompilerServices.AsyncMethodBuilder(
+            typeof(System.Runtime.CompilerServices.PoolingAsyncValueTaskMethodBuilder<>))]
+#endif
         public async ValueTask<IServiceResponse> SendRequestAsync(
             IServiceRequest request,
             int timeout,