notifications on approvals and turn end (#3329)

uses OSC 9 to notify when a turn ends or approval is required. won't
work in vs code or terminal.app but iterm2/kitty/wezterm supports it :)

codex-rs/tui/src/app.rs

@@ -177,6 +177,7 @@ impl App {
                     self.chat_widget.handle_paste(pasted);
                 }
                 TuiEvent::Draw => {
+                    self.chat_widget.maybe_post_pending_notification(tui);
                     if self
                         .chat_widget
                         .handle_paste_burst_tick(tui.frame_requester())