Fix logging/setLevel to return empty hash per MCP specification

This PR fixes `logging/setLevel` to return empty hash per MCP specification.
The MCP specification requires `logging/setLevel` to return an empty result
on success, not nil or the `LoggingMessageNotification` object.

Specification reference:
https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/logging

Fixes #229.

lib/mcp/server.rb

@@ -362,6 +362,8 @@ module MCP
       end
 
       @logging_message_notification = logging_message_notification
+
+      {}
     end
 
     def list_tools(request)