Closes https://github.com/zed-industries/zed/issues/41094

This PR adds the `agent_buffer_font_size` settings to be scaled in the
same proportion as the agent panel's UI font size, which was the only
setting that was being accounted for. This is something that I forgot to
do when we broke down the agent panel's font size controls into these
two values.

Release Notes:

- agent: Fixed an issue where the agent panel's buffer and UI font size
wouldn't scale proportionally.