Spring AI DeepSeekAssistantMessage.builder() returned an unexpected type

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

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

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