ManagedSession: release the reader lock from a pooled state machine

ExitReaderOnCompletion registered the release as a continuation on the
service call's task, so the caller's await was the second continuation
and allocated the task's continuation list (~80 B per request). On .NET 6
and later the call is awaited in a pooled state machine that releases the
lock before its own result completes; net48 keeps the continuation.

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

src/Opc.Ua.Client/Utils/AsyncReaderWriterLock.cs

@@ -28,6 +28,7 @@
  * ======================================================================*/
 
 using System;
+using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -96,7 +97,9 @@ namespace Opc.Ua.Client
         /// </summary>
         public AsyncReaderWriterLock()
         {
+#if !NET6_0_OR_GREATER
             m_releaseReader = ReleaseReader;
+#endif
         }
 
         /// <summary>
@@ -187,13 +190,16 @@ namespace Opc.Ua.Client
         /// task for the same result.
         /// </summary>
         /// <remarks>
-        /// The release is the first continuation registered on the operation's
-        /// task and runs synchronously when it completes, so the lock is normally
-        /// free again by the time an awaiter of the returned task resumes, as with
-        /// a <c>using</c> block around the awaited operation. Only if the task
-        /// completes while the continuation is being registered does the release
-        /// run on the thread pool instead, shortly after. The delegate is cached,
-        /// so this allocates nothing beyond the task's continuation list.
+        /// The lock is free again by the time an awaiter of the returned task
+        /// resumes, as with a <c>using</c> block around the awaited operation.
+        /// On .NET 6 and later the returned task awaits the operation in a pooled
+        /// state machine, the only continuation of the operation, so this
+        /// allocates nothing in the steady state; the returned task must be
+        /// awaited once, like any value task. Otherwise the release is the first
+        /// continuation registered on the operation's task and the caller's await
+        /// the second, which allocates the task's continuation list; only if the
+        /// task completes while the release is being registered does it run on
+        /// the thread pool, shortly after.
         /// </remarks>
         /// <typeparam name="T">The result of the operation.</typeparam>
         public ValueTask<T> ExitReaderOnCompletion<T>(ValueTask<T> operation)
@@ -203,11 +209,34 @@ namespace Opc.Ua.Client
                 ReleaseReader();
                 return operation;
             }
+#if NET6_0_OR_GREATER
+            return ExitReaderAfterAsync(operation);
+#else
             Task<T> task = operation.AsTask();
             task.ConfigureAwait(false).GetAwaiter().UnsafeOnCompleted(m_releaseReader);
             return new ValueTask<T>(task);
+#endif
         }
 
+#if NET6_0_OR_GREATER
+        /// <summary>
+        /// Awaits <paramref name="operation"/> and releases the reader lock.
+        /// </summary>
+        /// <typeparam name="T">The result of the operation.</typeparam>
+        [AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder<>))]
+        private async ValueTask<T> ExitReaderAfterAsync<T>(ValueTask<T> operation)
+        {
+            try
+            {
+                return await operation.ConfigureAwait(false);
+            }
+            finally
+            {
+                ReleaseReader();
+            }
+        }
+#endif
+
         /// <summary>
         /// Asynchronously acquires the writer lock. Excludes both
         /// other writers and all readers for the duration the lock
@@ -390,7 +419,9 @@ namespace Opc.Ua.Client
             }
         }
 
+#if !NET6_0_OR_GREATER
         private readonly Action m_releaseReader;
+#endif
         private bool m_isDisposed;
         private readonly SemaphoreSlim m_writer = new(1, 1);
         private readonly Lock m_state = new();