Spring-ai fix refusal field in ChatCompletion model

fix exception with field refusal in ChatCompletion model

https://github.com/spring-projects/spring-ai/issues/1178

Comment From: tzolov

Thank you for the contribution @TarasVovk669

Comment From: tzolov

Extended, rebased, squashed and merged at 866b262cddb10a54b832b9cd1ca25fa3c698dee4