[python]: Revisit `content_type` determination

[cpsievert]

Once #62 is done, we should revisit the determination of ClientMessage's content_type.

shinychat/pkg-py/src/shinychat/_chat.py

Line 988 in 82f43c6

content_type = "html" if isinstance(content, HTML) else "markdown"

And, in fact, that determination should probably be made by ChatMessage.

Note also that after #119, ChatMessage now essentially checks whether content_type is "html" and, if so, wraps it in a "literal HTML" wrapper. I think we could, instead, set content_type="html" (to avoid going through the markdown parser) and remove the need for the "literal HTML" wrapper?