Polish diff for the edit_file tool (#29911)

I added some padding to the editor, and removed the border around each
hunk as it would overlap in weird ways with the card container.

## Before

<img width="1148" alt="image"
src="https://github.com/user-attachments/assets/2018feaa-c847-4609-bc82-522660714b9a"
/>

## After

One Light:

<img width="1148" alt="image"
src="https://github.com/user-attachments/assets/4da1a4b6-0af2-4479-afcc-02da50178fd6"
/>

One Dark:

<img width="1148" alt="image"
src="https://github.com/user-attachments/assets/0168631d-7b76-4582-8174-c6e9c1297dc8"
/>


Release Notes:

- Improved displaying of diffs when the agent edits files.

crates/editor/src/editor.rs

@@ -517,6 +517,7 @@ pub enum SoftWrap {
 #[derive(Clone)]
 pub struct EditorStyle {
     pub background: Hsla,
+    pub horizontal_padding: Pixels,
     pub local_player: PlayerColor,
     pub text: TextStyle,
     pub scrollbar_width: Pixels,
@@ -531,6 +532,7 @@ impl Default for EditorStyle {
     fn default() -> Self {
         Self {
             background: Hsla::default(),
+            horizontal_padding: Pixels::default(),
             local_player: PlayerColor::default(),
             text: TextStyle::default(),
             scrollbar_width: Pixels::default(),
@@ -20352,6 +20354,7 @@ impl Render for Editor {
             &cx.entity(),
             EditorStyle {
                 background,
+                horizontal_padding: Pixels::default(),
                 local_player: cx.theme().players().local(),
                 text: text_style,
                 scrollbar_width: EditorElement::SCROLLBAR_WIDTH,