Add chat.messages(format='internal')

[wch]

This changes PR changes chat.messages() so that:

• A value for format is required. Previously the default value was MISSING.
• To get the internal/native format, the user calls it with chat.messages(format="internal") (instead of format=MISSING, which was the previously the default).

Previously, the default MISSING would result in it returning the messages in the internal ChatMessage format. However, this format is actually the least likely format that a user would want -- it is used internally in Shiny, but is not directly usable by any LLMs out there. I think that we don't want to have a default format which is not useful.

Note that this builds on PR #1530.

Also, I'm open to ideas other than "internal" for the name.

[cpsievert]

However, this format is actually the least likely format that a user would want -- it is used internally in Shiny, but is not directly usable by any LLMs out there.

That's not actually true. This format can be passed directly OpenAI, LangChain, Anthropic, Ollama, and probably others without modification. It won't pass a type checker, but that doesn't seem like reason enough to force everyone to have to specify a format.

Suggested change

	format: ProviderMessageFormat | Literal["internal"],
	format: ProviderMessageFormat | Literal["internal"] = "internal",

[wch]

Oh wait, I think I misunderstood what the default format was -- I thought it was the actual internal format that the Chat class uses, but that's not right. The internal format is StoredMessage, not ChatMessage. That's why I thought it wouldn't make sense to return that format by default.

So to summarize, prior to this PR, the default format was ChatMessage, which is sort of a lowest-common-denominator format that has role and content.

So it might make sense to just drop this PR entirely. That said, it could still be useful to be able to access the StoredMessages in the chat object. That's something I actually wanted to do when I was trying to get a handle on the sizes of messages being sent to the LLM (but then I got sidetracked before I went in depth on it).

[cpsievert]

Ah, I see. I'd be interested to know more about why you want access to StoredMessage.

I'll close this PR and create another one for the format="anthropic" thing

[cpsievert]

It's technically possible that messages2 is of length 0 here, so we'll need to be more careful about that.

from shiny import reactive
from shiny.express import ui

chat = ui.Chat(id="chat")
chat.ui()

@reactive.effect
def _():
    print(chat.messages(format="anthropic"))