Implement SEP-2663 Tasks Extension (#1579)

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

src/ModelContextProtocol.Core/Protocol/CancelTaskRequestParams.cs

@@ -0,0 +1,29 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the parameters for a <c>tasks/cancel</c> request to signal intent to cancel an in-progress task.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Cancellation is cooperative: the request signals intent, and the server decides whether and when to honor it.
+/// A server is not obligated to actually stop the work; it is only obligated to acknowledge the request.
+/// Eventual transition to <see cref="McpTaskStatus.Cancelled"/> is not guaranteed.
+/// </para>
+/// <para>
+/// The <c>notifications/cancelled</c> notification must not be used for task cancellation.
+/// </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 CancelTaskRequestParams : RequestParams
+{
+    /// <summary>
+    /// Gets or sets the identifier of the task to cancel.
+    /// </summary>
+    [JsonPropertyName("taskId")]
+    public required string TaskId { get; set; }
+}