assistant: Undo workflow step when buffer is discarded (#16465)

This fixes a weird bug:

1. Use `/workflow` in assistant
2. Have it generate a step that modifies a file
3. Either (a) select the step in the assistant and have it auto-insert
newlines (b) select "Transform" to have the step applied
4. Close the modified file in the editor ("Discard")
5. Re-open the file
6. BUG: the changes made by assistant are still there!

The reason for the bug is that the assistant keeps references to buffers
and they're not closed/reloaded when closed/reopened.

To fix the bug we now rollback the applied workflow steps when
discarding a buffer.

(This does *not* yet fix the issue where a workflow step inserts a new
buffer into the project/worktree that does not show up on the file
system yet but in `/file` and hangs around until Zed is closed.)


Release Notes:

- N/A

Co-authored-by: Bennet <bennet@zed.dev>

crates/assistant/src/assistant_panel.rs

@@ -2479,6 +2479,18 @@ impl ContextEditor {
         };
 
         let resolved_step = step.read(cx).resolution.clone();
+
+        if let Some(Ok(resolution)) = resolved_step.as_ref() {
+            for (buffer, _) in resolution.suggestion_groups.iter() {
+                let step_range = step_range.clone();
+                cx.subscribe(buffer, move |this, _, event, cx| match event {
+                    language::Event::Discarded => this.undo_workflow_step(step_range.clone(), cx),
+                    _ => {}
+                })
+                .detach();
+            }
+        }
+
         if let Some(existing_step) = self.workflow_steps.get_mut(&step_range) {
             existing_step.resolved_step = resolved_step;
         } else {