Spring-ai Fix where previous Assistant Messages were not being added to the parts list

The Google Gemini Vertex Chat Model was not adding previous Assistant Messages to the parts list - this results in a failure when submitting a call:

Results in: io.grpc.StatusRuntimeException: INVALID_ARGUMENT: Unable to submit request because it must include at least one parts field, which describes the prompt input. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini

Comment From: tzolov

This looks like a nasty bug. Thanks for catching it.

Comment From: tzolov

@dolukhanov , I added a test to reproduce and validate your patch: b5c7975facbb5cccc202ee3c04d5b2534f06c2a7