Update content according to latest spec (#513)

* Update content according to latest spec

* Add Title to a bunch of types

* Add _meta to lots of types

* Address feedback, more cleanup, fix NAOT test failure

src/ModelContextProtocol.Core/Protocol/CancelledNotification.cs

@@ -1,30 +0,0 @@
-using System.Text.Json.Serialization;
-
-namespace ModelContextProtocol.Protocol;
-
-/// <summary>
-/// Represents a notification indicating that a request has been cancelled by the client,
-/// and that any associated processing should cease immediately.
-/// </summary>
-/// <remarks>
-/// This class is typically used in conjunction with the <see cref="NotificationMethods.CancelledNotification"/>
-/// method identifier. When a client sends this notification, the server should attempt to
-/// cancel any ongoing operations associated with the specified request ID.
-/// </remarks>
-public sealed class CancelledNotification
-{
-    /// <summary>
-    /// Gets or sets the ID of the request to cancel.
-    /// </summary>
-    /// <remarks>
-    /// This must match the ID of an in-flight request that the sender wishes to cancel.
-    /// </remarks>
-    [JsonPropertyName("requestId")]
-    public RequestId RequestId { get; set; }
-
-    /// <summary>
-    /// Gets or sets an optional string describing the reason for the cancellation request.
-    /// </summary>
-    [JsonPropertyName("reason")]
-    public string? Reason { get; set; }
-}