Add missing McpException documentation to McpClient.GetTaskResultAsync

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/McpClient.Methods.cs

@@ -1073,6 +1073,7 @@ public async ValueTask<McpTask> GetTaskAsync(
     /// <returns>The raw JSON result of the task.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="taskId"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentException"><paramref name="taskId"/> is empty or composed entirely of whitespace.</exception>
+    /// <exception cref="McpException">The request failed or the server returned an error response.</exception>
     /// <remarks>
     /// This method sends a tasks/result request to the server, which will block until the task completes if it hasn't already.
     /// The server handles all polling logic internally.