Add a response target picker to /copy (#39997)

## What changed

- Open a picker for `/copy` with the whole response plus each fenced code block and blockquote from the latest response.
- Label code blocks by language, show content previews, and preserve source whitespace and nested quote Markdown when copying an individual target.
- Keep the picker from disturbing terminal scrollback or queued input, retain clipboard leases, and avoid writing copied content to the session log.

## Testing

- Add coverage for target extraction, picker navigation and cancellation, exact copied content, plan responses, queued input, scrollback preservation, clipboard failures, and session logging.

GitOrigin-RevId: 77a35bc99f4f45932d39914f9a709b30fe8cd7c2

codex-rs/tui/src/app/event_dispatch.rs

@@ -141,6 +141,9 @@ impl App {
                     self.chat_widget.maybe_send_next_queued_input();
                 }
             }
+            AppEvent::CopySelection { text, label } => {
+                self.chat_widget.copy_selection(text, label);
+            }
             AppEvent::ClearUi { name } => {
                 self.clear_terminal_ui(tui, /*redraw_header*/ false)?;
                 self.reset_app_ui_state_after_clear();