assistant: Make `/file` emit events as they occur (#19743)

This PR updates the `/file` command to emit its `SlashCommandEvent`s in
a way that can actually be streamed.

Previously it was buffering up all of the events and then returning them
all at once.

Note that we still don't yet support streaming in the context editor on
`main`, so there won't be any visible changes just yet.

Release Notes:

- N/A