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>

crates/markdown/Cargo.toml

@@ -20,16 +20,15 @@ test-support = [
 
 [dependencies]
 anyhow.workspace = true
-file_icons.workspace = true
 gpui.workspace = true
 language.workspace = true
 linkify.workspace = true
 log.workspace = true
 pulldown-cmark.workspace = true
+sum_tree.workspace = true
 theme.workspace = true
 ui.workspace = true
 util.workspace = true
-workspace.workspace = true
 workspace-hack.workspace = true
 
 [dev-dependencies]