agent: Introduce agent_buffer_font_size setting (#39468)

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.

crates/assistant_tools/src/edit_file_tool.rs

@@ -1003,7 +1003,7 @@ impl ToolCard for EditFileToolCard {
                 font_size: Some(
                     TextSize::Small
                         .rems(cx)
-                        .to_pixels(ThemeSettings::get_global(cx).agent_font_size(cx))
+                        .to_pixels(ThemeSettings::get_global(cx).agent_ui_font_size(cx))
                         .into(),
                 ),
                 ..TextStyleRefinement::default()