Spring-ai Refactor: Replace instanceof checks and switch expression

I refactored some code to improve readability: - Extracted the result of instanceof checks into variables - Refactored to use switch expressions instead of if-else blocks

Comment From: jitokim

you could include your name in the "author" field.
~~also, adding test cases to verify the functionality would make it even better.~~ (its already written. i have checked.) 👍

btw, a similar logic exists in BedrockProxyChatModel

Comment From: delvering17

Thank you for the feedback!

I also refactored the BedrockProxyChatModel. There was an unnecessary instanceof check, so I removed it. Additionally, I updated the previously refactored OpenAiChatModel to use swtich expression instead of switch-case statement.