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/assistant_tools/Cargo.toml

@@ -50,13 +50,13 @@ strsim.workspace = true
 task.workspace = true
 terminal.workspace = true
 terminal_view.workspace = true
+theme.workspace = true
 ui.workspace = true
 util.workspace = true
 web_search.workspace = true
 workspace-hack.workspace = true
 workspace.workspace = true
 zed_llm_client.workspace = true
-theme.workspace = true
 
 [dev-dependencies]
 client = { workspace = true, features = ["test-support"] }