Extend progress notification support (#163)

* Extend progress notification support

* Address feedback and fix test

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol/McpEndpointExtensions.cs

@@ -0,0 +1,34 @@
+using ModelContextProtocol.Protocol.Messages;
+using ModelContextProtocol.Utils;
+
+namespace ModelContextProtocol;
+
+/// <summary>Provides extension methods for interacting with an <see cref="IMcpEndpoint"/>.</summary>
+public static class McpEndpointExtensions
+{
+    /// <summary>Notifies the connected endpoint of progress.</summary>
+    /// <param name="endpoint">The endpoint issueing the notification.</param>
+    /// <param name="progressToken">The <see cref="ProgressToken"/> identifying the operation.</param>
+    /// <param name="progress">The progress update to send.</param>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    /// <returns>A task representing the completion of the operation.</returns>
+    /// <exception cref="ArgumentNullException"><paramref name="endpoint"/> is <see langword="null"/>.</exception>
+    public static Task NotifyProgressAsync(
+        this IMcpEndpoint endpoint,
+        ProgressToken progressToken,
+        ProgressNotificationValue progress, 
+        CancellationToken cancellationToken = default)
+    {
+        Throw.IfNull(endpoint);
+
+        return endpoint.SendMessageAsync(new JsonRpcNotification()
+        {
+            Method = NotificationMethods.ProgressNotification,
+            Params = new ProgressNotification()
+            {
+                ProgressToken = progressToken,
+                Progress = progress,
+            },
+        }, cancellationToken);
+    }
+}