History processor replaces message history

[AlexEnrique]

This PR closes #2095

Summary

• [BREAKING CHANGE]: history_processor's now replaces the message history with its result.
• Updates docs with warnings about new behavior and issues related to slicing the message history
• Added tests to test new behavior

Concerns

The new behavior also processes/replaces the current ModelRequest being sent to the LLM. This may lead to unexpected behaviors for users of the library, for example, the following history_processor must prepend the messages[-1:] in order to not remove the model request being sent.

def process_previous_answers(messages: list[ModelMessage]) -> list[ModelMessage]:
        # Keep the last message (last question) and add a new system prompt
        return messages[-1:] + [ModelRequest(parts=[SystemPromptPart(content='Processed answer')])]

Although this may be unexpected, changing it to keep the last model request as is may also be a surprise for users. This was in conflict with exiting tests and I though it would be better to keep in this way and let the library user handle this.

I've added a warning to the docs, but maybe I need to emphasize this more.

c.c: @Kludex

[hyperlint-ai]

PR Change Summary

Updated the message history processing behavior in the library, introducing breaking changes and enhanced documentation to address potential user concerns.

• Replaced message history with results from the history processor, introducing a breaking change.
• Updated documentation with warnings about new behavior and issues related to message history slicing.
• Added tests to validate the new message history processing behavior.

Modified Files

• docs/message-history.md

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[Wh1isper]

LGTM

[DouweM]

@AlexEnrique Thanks Alex! The code looks good to me but I'd like for @Kludex to have a look at the concern you mention in the description as he built the feature. He's out this week though, so expect a more careful review of the implications early next week!

[Kludex]

Thanks folks. I think we should add a note on our breaking changes page in the documentation as well.

[AlexEnrique]

@Kludex There is a warning in the beginning of the section Processing Message History. Isn't it enough?

I'll copy it here to save you a search:

!!! warning "History processors replace the message history"
History processors replace the message history in the state with the processed messages, including the new user prompt part.
This means that if you want to keep the original message history, you need to make a copy of it.

[Kludex]

I forgot this PR, I'll merge tomorrow