This adds `code_executions` as an array of code execution statuses to
chat messages. The intent of this data is to be displayed in a similar
manner as citations: at the bottom of the message, with buttons that open
a modal for more info. However, code execution data doesn't fit well in
citation modals, because they fundamentally differ in their formatting.
Code execution status includes the code that was run (which benefits from
being syntax-highlighted), and the output and generated files. This
differs from citations which are just list of document names and links.
Additionally, code execution is a process, whereas citations are only
emitted once. This is why code execution data uses an ID-based approach,
where each code execution instance is identified by a unique ID and can
be updated by emitting a new `code_execution` message with the same ID.
This allows the code execution status to be updated as code runs.
