agent: Render diffs for the edit file tool (#29234)

This PR implements the `ToolCard` for the edit file tool, which allow us
to display an editor with a diff in the thread view with the changes
performed by the model.

- [x] Fix buffer sometimes displaying empty
- [x] Stop buffer from scrolling together with the thread
- [x] Fix multibuffer header sometimes appearing
- [x] Fix buffer height issue
- [x] Implement "full height" expand button
- [x] Add "Jump To File" functionality
- [x] Polish and refine styles

Release Notes:

- agent: Added diff preview cards in the thread view for edits performed
by the agent.

---------

Co-authored-by: João Marcos <marcospb19@hotmail.com>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Agus Zubiaga <hi@aguz.me>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/eval/src/example.rs

@@ -278,7 +278,7 @@ impl ExampleContext {
 
         let message_count_before = self.app.update_entity(&self.agent_thread, |thread, cx| {
             thread.set_remaining_turns(iterations);
-            thread.send_to_model(model, cx);
+            thread.send_to_model(model, None, cx);
             thread.messages().len()
         })?;