systemMessage should come first index

Comment From: ThomasVitale

Thanks for your PR! We have an issue tracking how to improve the way system messages are handled in ChatClient: https://github.com/spring-projects/spring-ai/issues/873 I would suggest having a conversation there before making any change to make sure all use cases that have been identified are covered.

Comment From: markpollack

This is indeed an important issue, thanks for the PR. I am closing it as this topic requires a larger discussion and #873 will be used for tracking this going forward.