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.rs

@@ -287,6 +287,12 @@ pub(crate) enum AppEvent {
         destination: TranscriptExportDestination,
     },
 
+    /// Copy a picker selection while retaining its clipboard lease in the chat widget.
+    CopySelection {
+        text: Arc<str>,
+        label: String,
+    },
+
     /// Persist a submitted prompt in the cross-session message history.
     AppendMessageHistoryEntry {
         thread_id: ThreadId,