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

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

src/ModelContextProtocol.Core/Protocol/CreateTaskResult.cs

@@ -0,0 +1,28 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the response to a task-augmented request.
+/// </summary>
+/// <remarks>
+/// <para>
+/// When a client sends a request with a <c>task</c> parameter, the server immediately returns
+/// a <see cref="CreateTaskResult"/> containing the created task information instead of the
+/// normal result type. The actual result can be retrieved later via <c>tasks/result</c>.
+/// </para>
+/// <para>
+/// This type is returned for any task-augmented request including <c>tools/call</c>,
+/// <c>sampling/createMessage</c>, and <c>elicitation/create</c>.
+/// </para>
+/// </remarks>
+[Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+public sealed class CreateTaskResult : Result
+{
+    /// <summary>
+    /// Gets or sets the task data for the newly created task.
+    /// </summary>
+    [JsonPropertyName("task")]
+    public McpTask Task { get; set; } = null!;
+}