Gut mistakes made in python Chat

[cpsievert]

This PR removes some features in Chat that predate chatlas and add a lot of complexity to the implementation without much benefit.

For reference, here is the list:

• Chat(messages=) was removed. Use chat.ui(messages=) instead.
• Chat(tokenizer=) was removed. This was only relevant for .messages(token_limits=[]) which was also removed.
• Several parameters were removed from .messages(). This reflects an overall change approach for maintaining the conversation history sent to the LLM -- Chat should no longer be responsible for maintaining this conversation state, and instead, another stateful object (perhaps the one provided by chatlas, Pydantic AI, LangChain, etc.) should be used to maintain this state. That said, .messages() can still be useful if you want a reflection of the message state on the client.
• The transform_user_input and transform_assistant_response decorators were removed. This change also reflects the new recommendation of using another stateful object to submit input and retain conversation history.
• .user_input()'s transform parameter was removed (as a consequence of the previous point).
• The already deprecated .set_user_message() method was officially removed (in favor of .update_user_input())

NOTE: Although these features are removed here, we should do a py-shiny release that deprecates these features first, then removes them full-stop when we move to re-export shinychat in shiny.

Follow up

• Create shiny PR to deprecate these features
• Possibly externalize the _chat_normalize.py logic into a more explicit API for transforming provider-specific completion objects into a ChatMessage/ChatMessageDict. That way .append_message() et al. can have a stronger message type.
• When we eventually go to re-export shinychat in shiny, remove the "3rd party" LLM dependencies from shiny's pyproject.toml

[gadenbuie]

@cpsievert Can you update the base branch to main and re-open the PR?

[cpsievert]

I just opened up a new one in #62