Add missing [Experimental] attributes for MCP tasks properties

Add [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
to three task-related properties that were accidentally missing it:
- CallToolResult.Task (McpTask?)
- ClientCapabilities.Tasks (McpTasksCapability?)
- ServerCapabilities.Tasks (McpTasksCapability?)

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/CallToolResult.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
@@ -72,6 +73,7 @@ public sealed class CallToolResult : Result
     /// (<see cref="Content"/>, <see cref="StructuredContent"/>, <see cref="IsError"/>) may not be populated.
     /// The actual tool result can be retrieved later via <c>tasks/result</c>.
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
     [JsonPropertyName("task")]
     public McpTask? Task { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Server;
@@ -80,6 +81,7 @@ public sealed class ClientCapabilities
     /// See <see cref="McpTasksCapability"/> for details on configuring which operations support tasks.
     /// </para>
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
     [JsonPropertyName("tasks")]
     public McpTasksCapability? Tasks { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Server;
 
@@ -79,6 +80,7 @@ public sealed class ServerCapabilities
     /// See <see cref="McpTasksCapability"/> for details on configuring which operations support tasks.
     /// </para>
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
     [JsonPropertyName("tasks")]
     public McpTasksCapability? Tasks { get; set; }
 }