Spring-ai Add the default model to the initial Vertex AI Gemini autoconfiguration chat properties

I'm not sure if there is a reason for it, but Vertex AI Gemini basic autoconfiguration didn't carry a default model, unlike some other implementations (like OpenAI's).

Setting the default model makes it a little bit more plugandplay'able.

Comment From: tzolov

Makes sense. Thanks @zucchivan

Comment From: tzolov

can you confirm that you've run ./mvnw spring-javaformat:apply ?

Comment From: zucchivan

Yups, it's done!