feat: enable contents_record/replay to handle lists (#895)

[hopessugar]

Closes #895.

This PR updates contents_record() and contents_replay() to automatically handle lists of turns/objects. This simplifies saving and loading chat history by removing the need for manual lapply calls, as requested.

[gadenbuie]

Thanks, but unfortunately, the fix for this issue is going to be more complicated. We'd need to introduce an intermediate class to ensure the general type-safety of contents_replay() and contents_record(). I mentioned this in #895 (comment) but could have been clearer that we're not wanting to follow the approach initially mentioned in the issue. I kept that code in case it's useful to someone, but it's not intended to be incorporated into ellmer as such.