Conversation get_messages gets code_message

[jakethekoenig]

Frequently in the codebase people will go to conversation for the messages and then independently get the code_message. It's simpler if the conversation handles that.

get_messages is also changed to accept a system prompt argument instead of having a include_system_prompt parameter to simplify agent's calling of it. If callers genuinely want no system prompt they can pass [].

Move responsibility to raise if context too big from code_context to conversation.

Pull Request Checklist

• [x] Documentation has been updated, or this change doesn't require that

[mentatbot[bot]]

MENTAT CODE REVIEW IN BETA. Please Reply with feedback

This pull request introduces several key improvements, notably making various methods asynchronous to align with the overall async architecture of the application. The changes are well thought out and implemented, ensuring consistency and efficiency in message handling and command execution. It's particularly commendable how the changes have been propagated throughout the codebase, including updating tests to reflect the new asynchronous behavior. Overall, this PR represents a solid step forward in maintaining and improving the codebase's quality and functionality.