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/multi_buffer/src/multi_buffer.rs

@@ -106,6 +106,7 @@ pub enum Event {
     Saved,
     FileHandleChanged,
     Closed,
+    Discarded,
     DirtyChanged,
     DiagnosticsUpdated,
 }
@@ -1691,6 +1692,7 @@ impl MultiBuffer {
             language::Event::Reparsed => Event::Reparsed(buffer.read(cx).remote_id()),
             language::Event::DiagnosticsUpdated => Event::DiagnosticsUpdated,
             language::Event::Closed => Event::Closed,
+            language::Event::Discarded => Event::Discarded,
             language::Event::CapabilityChanged => {
                 self.capability = buffer.read(cx).capability();
                 Event::CapabilityChanged