Implement MCP task support (SEP-1686) (#1170)

Co-authored-by: Stephen Halter <halter73@gmail.com>

src/ModelContextProtocol.Core/Protocol/GetTaskPayloadRequestParams.cs

@@ -0,0 +1,27 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the parameters for a tasks/result request to retrieve the result of a completed task.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This request blocks until the task reaches a terminal status (<see cref="McpTaskStatus.Completed"/>,
+/// <see cref="McpTaskStatus.Failed"/>, or <see cref="McpTaskStatus.Cancelled"/>).
+/// </para>
+/// <para>
+/// The result structure matches the original request type (e.g., <see cref="CallToolResult"/> for tools/call).
+/// This is distinct from the initial <see cref="Result"/> response, which contains only task data.
+/// </para>
+/// </remarks>
+[Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+public sealed class GetTaskPayloadRequestParams : RequestParams
+{
+    /// <summary>
+    /// Gets or sets the unique identifier of the task whose result to retrieve.
+    /// </summary>
+    [JsonPropertyName("taskId")]
+    public required string TaskId { get; set; }
+}