Link to cited code blocks (#28217)

<img width="612" alt="Screenshot 2025-04-06 at 9 59 41 PM"
src="https://github.com/user-attachments/assets/3a996b4a-ef5c-4ca6-bd16-3b180b364a3a"
/>

Release Notes:

- Agent panel now shows links to relevant source code files above code
blocks.

Cargo.lock

@@ -8355,6 +8355,7 @@ dependencies = [
  "anyhow",
  "assets",
  "env_logger 0.11.8",
+ "file_icons",
  "gpui",
  "language",
  "languages",
@@ -8366,6 +8367,7 @@ dependencies = [
  "theme",
  "ui",
  "util",
+ "workspace",
  "workspace-hack",
 ]