Spring AI ZhiPuAiAssistantMessage.builder() returned an unexpected type

This is an issue similar to #4452.

Bug description Since we recently added support for the Builder pattern to AssistantMessage, this causes classes like ZhiPuAiAssistantMessage—which inherit from AssistantMessage—to also inherit the builder of AssistantMessage, which is clearly problematic.

Steps to reproduce Try using ZhiPuAiAssistantMessage.builder().build(), and you'll get an AssistantMessage instead of a ZhiPuAiAssistantMessage.

Expected behavior ZhiPuAiAssistantMessage.builder().build() should return a ZhiPuAiAssistantMessage.