withSystemInstruction returns a new model

[turchinc]

see also #1030

[turchinc]

I tested this locally by replacing just the gemini jar

    implementation 'org.springframework.ai:spring-ai-vertex-ai-gemini'
    //TODO replace when bug is fixed 
    implementation fileTree(dir: 'libs', include: ['*.jar'])

and the results are now as expected

You are a helpful assistant, providing guidance based on the information provided below. Always return as JSON format:
Format example:
{"response": "This is the useful response from the helpful assistant", "link": "https://link-for-more-information"}
What is the URL for the google search engine?

{"response": "The URL for the Google search engine is https://www.google.com/", "link": "https://www.google.com/"}

[tzolov]

Thanks @turchinc, I've added some additional changes to the PR to update the default model and improve the message handling cohesion.
Rebased, squashed and merged at f07e391