Add Elicitation skeleton

The MCP 2025-06-18 specification includes a feature called Elicitation:
https://modelcontextprotocol.io/specification/2025-06-18/client/elicitation

Like Log Level, an implementation is expected to be provided in the near future.
This PR adds Elicitation as a skeleton implementation for now.

lib/mcp/server.rb

@@ -80,6 +80,7 @@ module MCP
         Methods::RESOURCES_UNSUBSCRIBE => ->(_) {},
         Methods::COMPLETION_COMPLETE => ->(_) {},
         Methods::LOGGING_SET_LEVEL => ->(_) {},
+        Methods::ELICITATION_CREATE => ->(_) {},
       }
       @transport = transport
     end