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/agent/src/message_editor.rs

@@ -509,13 +509,7 @@ impl MessageEditor {
             }))
     }
 
-    fn render_editor(
-        &self,
-        font_size: Rems,
-        line_height: Pixels,
-        window: &mut Window,
-        cx: &mut Context<Self>,
-    ) -> Div {
+    fn render_editor(&self, window: &mut Window, cx: &mut Context<Self>) -> Div {
         let thread = self.thread.read(cx);
         let model = thread.configured_model();
 
@@ -621,6 +615,10 @@ impl MessageEditor {
                             .when(is_editor_expanded, |this| this.h_full())
                             .child({
                                 let settings = ThemeSettings::get_global(cx);
+                                let font_size = TextSize::Small
+                                    .rems(cx)
+                                    .to_pixels(settings.agent_font_size(cx));
+                                let line_height = settings.buffer_line_height.value() * font_size;
 
                                 let text_style = TextStyle {
                                     color: cx.theme().colors().text,
@@ -1329,15 +1327,14 @@ impl Render for MessageEditor {
         let action_log = self.thread.read(cx).action_log();
         let changed_buffers = action_log.read(cx).changed_buffers(cx);
 
-        let font_size = TextSize::Small.rems(cx);
-        let line_height = font_size.to_pixels(window.rem_size()) * 1.5;
+        let line_height = TextSize::Small.rems(cx).to_pixels(window.rem_size()) * 1.5;
 
         v_flex()
             .size_full()
             .when(changed_buffers.len() > 0, |parent| {
                 parent.child(self.render_changed_buffers(&changed_buffers, window, cx))
             })
-            .child(self.render_editor(font_size, line_height, window, cx))
+            .child(self.render_editor(window, cx))
             .children({
                 let usage_callout = self.render_usage_callout(line_height, cx);