feat: open prompt in configured external editor (#7606)

Add `ctrl+g` shortcut to enable opening current prompt in configured
editor (`$VISUAL` or `$EDITOR`).


- Prompt is updated with editor's content upon editor close.
- Paste placeholders are automatically expanded when opening the
external editor, and are not "recompressed" on close
- They could be preserved in the editor, but it would be hard to prevent
the user from modifying the placeholder text directly, which would drop
the mapping to the `pending_paste` value
- Image placeholders stay as-is
- `ctrl+g` explanation added to shortcuts menu, snapshot tests updated



https://github.com/user-attachments/assets/4ee05c81-fa49-4e99-8b07-fc9eef0bbfce

codex-rs/tui/src/chatwidget/tests.rs

@@ -407,6 +407,7 @@ async fn make_chatwidget_manual(
         last_rendered_width: std::cell::Cell::new(None),
         feedback: codex_feedback::CodexFeedback::new(),
         current_rollout_path: None,
+        external_editor_state: ExternalEditorState::Closed,
     };
     (widget, rx, op_rx)
 }