Add a callback for receiving stderr output (#525)

* Add a callback for receiving stderr output

* Add asserts to test

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -146,6 +146,8 @@ public sealed partial class StdioClientTransport : IClientTransport
                         stderrRollingLog.Enqueue(data);
                     }
 
+                    _options.StandardErrorLines?.Invoke(data);
+
                     LogReadStderr(logger, endpointName, data);
                 }
             };

src/ModelContextProtocol.Core/Client/StdioClientTransportOptions.cs

@@ -69,4 +69,9 @@ public sealed class StdioClientTransportOptions
     /// </para>
     /// </remarks>
     public TimeSpan ShutdownTimeout { get; set; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>
+    /// Gets or sets a callback that is invoked for each line of stderr received from the server process.
+    /// </summary>
+    public Action<string>? StandardErrorLines { get; set; }
 }