Keep interrupted prompts in conversation history (#33198)

## What changed

- Leave an output-free interrupted prompt in the transcript and open a blank
  composer for the user's next instruction.
- Use the normal interruption notice for both `Esc` and `Ctrl-C` interrupts.
- Track safety-buffering prompts separately so failed retries can still restore
  the prompt, including after switching between threads.

## Testing

- Cover output-free `Esc` and `Ctrl-C` interrupts, failed safety-buffer retries,
  and restoration of safety-buffer state from thread snapshots.

GitOrigin-RevId: 30b368d00dfdb981d06d4c95ce3f252389b7917c

codex-rs/tui/src/app_event.rs

@@ -259,9 +259,6 @@ pub(crate) enum AppEvent {
     /// bubbling channels through layers of widgets.
     CodexOp(AppCommand),
 
-    /// Restore an output-free interrupted turn into the composer and roll it back.
-    RestoreCancelledTurn(UserMessage),
-
     /// Approve one retry of a recent auto-review denial selected in the TUI.
     ApproveRecentAutoReviewDenial {
         thread_id: ThreadId,