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/Root.cs

@@ -12,7 +12,7 @@ namespace ModelContextProtocol.Protocol;
 /// Roots provide a hierarchical structure for organizing and accessing resources within the protocol.
 /// Each root has a URI that uniquely identifies it and optional metadata like a human-readable name.
 /// </remarks>
-public class Root
+public sealed class Root
 {
     /// <summary>
     /// Gets or sets the URI of the root.
@@ -32,6 +32,6 @@ public class Root
     /// <remarks>
     /// This is reserved by the protocol for future use.
     /// </remarks>
-    [JsonPropertyName("meta")]
+    [JsonPropertyName("_meta")]
     public JsonElement? Meta { get; init; }
 }