Fix CreateMessageRequestParams.Metadata type from JsonElement? to JsonObject?

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

Author: Copilot

src/ModelContextProtocol.Core/Protocol/CreateMessageRequestParams.cs

@@ -1,5 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
-using System.Text.Json;
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;
@@ -68,7 +68,7 @@ public sealed class CreateMessageRequestParams : RequestParams
     /// </para>
     /// </remarks>
     [JsonPropertyName("metadata")]
-    public JsonElement? Metadata { get; set; }
+    public JsonObject? Metadata { get; set; }
 
     /// <summary>
     /// Gets or sets the server's preferences for which model to select.