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/workspace/src/pane.rs

@@ -1500,8 +1500,13 @@ impl Pane {
                         })?;
                         match answer {
                             Ok(0) => {}
-                            Ok(1) => return Ok(true), // Don't save this file
-                            _ => return Ok(false),    // Cancel
+                            Ok(1) => {
+                                // Don't save this file
+                                pane.update(cx, |_, cx| item.discarded(project, cx))
+                                    .log_err();
+                                return Ok(true);
+                            }
+                            _ => return Ok(false), // Cancel
                         }
                     } else {
                         return Ok(false);