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/chatwidget.rs

@@ -991,13 +991,6 @@ impl ChatWidget {
         }
     }
 
-    /// Record or update the raw markdown for the current agent turn.
-    fn record_agent_markdown(&mut self, message: &str) {
-        if !message.is_empty() {
-            self.transcript.record_agent_markdown(message.to_string());
-        }
-    }
-
     pub(crate) fn open_feedback_note(
         &mut self,
         category: crate::app_event::FeedbackCategory,