Spring-ai Accept String answer in BasicEvaluationTest instead of ChatResponse

Per a discussion with @markpollack, perhaps the evaluateQuestionAndAnswer() method in BasicEvaluationTest should accept a String instead of a ChatResponse.

By doing so, this enables tests to be written against services whose implementations are based on Spring AI (or potentially any AI/non-AI implementation) and use the evaluation test to evaluate the response, rather than requiring that the test work with a ChatResponse, which would require the service to expose how it's implemented.

This PR makes that change.

Comment From: markpollack

merged in 9956367fc842334dffdf2c3cfa078c9255895b97 also added not null check to question method argument.