`DeepSeekAssistantMessage.builder()` returned an unexpected type

[sunyuhan1998]

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.

[ericbottard]

Implement via #4453