Closes https://github.com/zed-industries/zed/issues/37882

Previously, every piece of text in the agent panel was controlled by
`agent_font_size`. Although it is nice to only have one setting to tweak
that, it could be a bit misleading particularly because we use
monospaced and sans-serif fonts for different elements in the panel. Any
editor/textarea in the panel, whehter it is the main message editor or
the previous message editor, uses the buffer font. Therefore, I think it
is reasonable to expect that tweaking `buffer_font_size` would also
change the agent panel's usage of buffer fonts.

With this change, regular buffers and the agent panel's message editor
will always have the same size.

Release Notes:

- agent: Made the agent panel's textarea font size follow the font size
of regular buffers. They're now both controlled by the
`buffer_font_size` setting.