Previously, we forgot to associate the `Markdown` entity to
`MarkdownElement` during `prepaint`. This caused calls to
`Context<Markdown>::notify` to not invalidate the view cache, which
meant we would have to wait for some other invalidation before seeing
the results of that initial notify.

Release Notes:

- Improved responsiveness of mouse interactions with the agent panel.

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>