Wrong normalizer used in tests

> @glengemann This seems to be a copy/paste error. This "OpenAi" test currently uses the "Gemini" `MessageBagNormalizer`. 

 _Originally posted by @petski in [e621538](https://github.com/symfony/ai/commit/e621538c419483406e910d917fe6fe7cf3270806#r164941544)_