agent: Add headers for code blocks (#28253)

<img width="639" alt="image"
src="https://github.com/user-attachments/assets/1fd51387-cbdc-474d-b1a3-3d0201f3735a"
/>


Release Notes:

- N/A

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

Cargo.lock

@@ -8354,7 +8354,6 @@ dependencies = [
  "anyhow",
  "assets",
  "env_logger 0.11.8",
- "file_icons",
  "gpui",
  "language",
  "languages",
@@ -8363,10 +8362,10 @@ dependencies = [
  "node_runtime",
  "pulldown-cmark 0.12.2",
  "settings",
+ "sum_tree",
  "theme",
  "ui",
  "util",
- "workspace",
  "workspace-hack",
 ]