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/bottom_pane/snapshots/codex_tui__bottom_pane__command_popup__tests__command_popup_default_items.snap

@@ -25,7 +25,7 @@ expression: commands
 /compact - summarize conversation to prevent hitting the context limit
 /agents - view and switch between all active agent sessions
 /side - start a side conversation in an ephemeral fork
-/copy - copy last response as markdown
+/copy - copy the last response, code block, or quote
 /export - export the conversation as markdown
 /raw - toggle raw scrollback mode for copy-friendly terminal selection
 /diff - show git diff (including untracked files)