Refactor the `extract_contents_message` function

This function is overly complex and is difficult to follow, especially given all the possible permutations that can be passed in as `content`. This has led to edge cases

Recommend addressing this before tackling something like [PY-2289](https://linear.app/getsentry/issue/PY-2289/improve-processing-of-content-argument-thats-passed-in-to-generate)