Closes https://github.com/zed-industries/zed/issues/39406
Follow up to https://github.com/zed-industries/zed/pull/38726

This PR introduces the `agent_buffer_font_size` setting and renames
`agent_font_size` to `agent_ui_font_size`. This allows whoever wants
`buffer_font_size` and `agent_buffer_font_size` to match, as well as
folks who want a slightly smaller size only in the agent panel (which...
also looks just better by default!).

Release Notes:

- agent: Introduced the `agent_buffer_font_size` setting and renamed
`agent_font_size` to `agent_ui_font_size`, allowing for granular buffer
font size control in the agent panel vs. regular editors.