Replace `rich_text` with `markdown` in `assistant2` (#11650)

We don't implement copy yet but it should be pretty straightforward to
add.


https://github.com/zed-industries/zed/assets/482957/6b4d7c34-de6b-4b07-aed9-608c771bbbdb

/cc: @rgbkrk @maxbrunsfeld @maxdeviant 

Release Notes:

- N/A