Add improved support for chat history message insertion (namely tools) + documentation

[willbakst]

Description

Right now users need to manually create the tools even though all of the information is generally already present in the tool. A method like tool.message_param would make this far more convenient (similar to how we can already to response.message.model_dump() for the assistant message.

We may also want to provide additional convenience, something like response.user_message_param, for inserting the user message back in. This one likely needs a little more thought than the tool call message feature.