Moved if directive to common extension class to avoid scattering directives in implementations.

Author: Tyler-R-Kendrick

src/ModelContextProtocol/Shared/McpSession.cs

@@ -397,11 +397,7 @@ public async Task<JsonRpcResponse> SendRequestAsync(JsonRpcRequest request, Canc
         finally
         {
             _pendingRequests.TryRemove(request.Id, out _);
-#if NETSTANDARD2_0
-            registration.Dispose();
-#else
             await registration.DisposeAsync().ConfigureAwait(false);
-#endif
             FinalizeDiagnostics(activity, startingTimestamp, durationMetric, ref tags);
         }
     }

src/ModelContextProtocol/Shared/TargetFrameworkExtensions.cs

@@ -0,0 +1,19 @@
+#if NETSTANDARD2_0
+
+using System.Diagnostics;
+
+/// <summary>
+/// Only used for .NET Standard 2.0.
+/// This class is a workaround for the lack of DisposeAsync.
+/// This allows us to reduce the number of #if directives in the code.
+/// </summary>
+internal static class TargetFrameworkExtensions
+{
+    public static Task DisposeAsync(this CancellationTokenRegistration registration)
+    {
+        registration.Dispose();
+        return Task.CompletedTask;
+    }
+}
+
+#endif