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

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

src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs

@@ -65,4 +65,20 @@ public sealed class ServerCapabilities
     /// </summary>
     [JsonPropertyName("completions")]
     public CompletionsCapability? Completions { get; set; }
+
+    /// <summary>
+    /// Gets or sets a server's tasks capability for supporting task-augmented requests.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The tasks capability enables clients to augment their requests with tasks for long-running
+    /// operations. When present, clients can request that certain operations (like tool calls)
+    /// execute asynchronously, with the ability to poll for status and retrieve results later.
+    /// </para>
+    /// <para>
+    /// See <see cref="McpTasksCapability"/> for details on configuring which operations support tasks.
+    /// </para>
+    /// </remarks>
+    [JsonPropertyName("tasks")]
+    public McpTasksCapability? Tasks { get; set; }
 }