Clean up namespaces (#410)

No meaningful code changes, just lots of namespace shuffling to have fewer namespaces and to put things in better namespaces.

src/ModelContextProtocol/Protocol/CancelledNotification.cs

@@ -0,0 +1,30 @@
+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; }
+}