Remove dead code in handle_regular_request

Notifications never reach `handle_regular_request` because `handle_post`
checks `notification?(body)` first and returns 202 via `handle_accepted`.
Since only notifications cause `handle_json` to return `nil`, `response`
is always non-`nil` here.

Author: koic

lib/mcp/server/transports/streamable_http_transport.rb

@@ -263,25 +263,18 @@ def handle_regular_request(body_string, session_id)
             end
           end
 
-          response = @server.handle_json(body_string) || ""
+          response = @server.handle_json(body_string)
 
           # Stream can be nil since stateless mode doesn't retain streams
           stream = get_session_stream(session_id) if session_id
 
           if stream
             send_response_to_stream(stream, response, session_id)
-          elsif response.nil? && notification_request?(body_string)
-            [202, { "Content-Type" => "application/json" }, [response]]
           else
             [200, { "Content-Type" => "application/json" }, [response]]
           end
         end
 
-        def notification_request?(body_string)
-          body = parse_request_body(body_string)
-          body.is_a?(Hash) && body["method"].start_with?("notifications/")
-        end
-
         def get_session_stream(session_id)
           @mutex.synchronize { @sessions[session_id]&.fetch(:stream, nil) }
         end