Implement SEP-2663 Tasks Extension (#1579)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/ModelContextProtocol.Core/Protocol/CancelTaskResult.cs

@@ -0,0 +1,22 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the result of a <c>tasks/cancel</c> request. This is an empty acknowledgement.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The server acknowledges the request with an empty result. Cancellation processing is
+/// eventually consistent — the task's observable status may remain <see cref="McpTaskStatus.Working"/>
+/// after the ack, and may ultimately reach a terminal status other than <see cref="McpTaskStatus.Cancelled"/>
+/// if the work finished before cancellation could take effect.
+/// </para>
+/// <para>
+/// See the <see href="https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/seps/2663-tasks-extension.md">SEP-2663</see>
+/// specification for details.
+/// </para>
+/// </remarks>
+public sealed class CancelTaskResult : Result
+{
+}