Suppress redundant attributes in Resources

According to the specification, the following attributes are optional,
so using `compact` ensures consistency.

- `description`: Optional description
- `mimeType`: Optional MIME type
- `size`: Optional size in bytes

https://modelcontextprotocol.io/specification/2025-03-26/server/resources#resource

Author: koic

lib/mcp/resource.rb

@@ -18,7 +18,7 @@ def to_h
         name: @name,
         description: @description,
         mimeType: @mime_type,
-      }
+      }.compact
     end
   end
 end