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/language/src/buffer.rs

@@ -332,6 +332,8 @@ pub enum Event {
     CapabilityChanged,
     /// The buffer was explicitly requested to close.
     Closed,
+    /// The buffer was discarded when closing.
+    Discarded,
 }
 
 /// The file associated with a buffer.
@@ -827,6 +829,12 @@ impl Buffer {
         cx.notify();
     }
 
+    /// This method is called to signal that the buffer has been discarded.
+    pub fn discarded(&mut self, cx: &mut ModelContext<Self>) {
+        cx.emit(Event::Discarded);
+        cx.notify();
+    }
+
     /// Reloads the contents of the buffer from disk.
     pub fn reload(
         &mut self,