Allow the agent panel font size to be customized (#29954)

You can set `agent_font_size` as a top-level settings key. You can also
use `zed::IncreaseBufferFontSize` and `zed::DecreaseBufferFontSize` and
`zed::ResetBufferFontSize` the agent panel is focused via the standard
bindings to adjust the agent font size. In the future, it might make
sense to rename these actions to be more general since "buffer" is now a
bit of a misnomer. 🍐'd with @mikayla-maki

Release Notes:

- N/A

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/assistant_tools/src/edit_file_tool.rs

@@ -540,7 +540,6 @@ impl ToolCard for EditFileToolCard {
             });
 
         let (editor, editor_line_height) = self.editor.update(cx, |editor, cx| {
-            let ui_font_size = ThemeSettings::get_global(cx).ui_font_size(cx);
             let line_height = editor
                 .style()
                 .map(|style| style.text.line_height_in_pixels(window.rem_size()))
@@ -558,7 +557,10 @@ impl ToolCard for EditFileToolCard {
                         font_family: settings.buffer_font.family.clone(),
                         font_features: settings.buffer_font.features.clone(),
                         font_fallbacks: settings.buffer_font.fallbacks.clone(),
-                        font_size: ui_font_size.into(),
+                        font_size: TextSize::Small
+                            .rems(cx)
+                            .to_pixels(settings.agent_font_size(cx))
+                            .into(),
                         font_weight: settings.buffer_font.weight,
                         line_height: relative(settings.buffer_line_height.value()),
                         ..Default::default()