Add Content::EmbeddedResource class for embedded resource content type

The MCP spec defines an embedded resource content type, but the SDK had
no corresponding class. Add `Content::EmbeddedResource` that wraps a
resource object and serializes it via `to_h`.

https://modelcontextprotocol.io/specification/2025-11-25/server/tools#embedded-resources

lib/mcp/content.rb

@@ -42,5 +42,18 @@ module MCP
         { data: data, mimeType: mime_type, annotations: annotations, type: "audio" }.compact
       end
     end
+
+    class EmbeddedResource
+      attr_reader :resource, :annotations
+
+      def initialize(resource, annotations: nil)
+        @resource = resource
+        @annotations = annotations
+      end
+
+      def to_h
+        { resource: resource.to_h, annotations: annotations, type: "resource" }.compact
+      end
+    end
   end
 end