Follow up issues of `postBanner/Footer`

Caused by https://github.com/rolldown/rolldown/pull/7583

- [ ] Incorrect sourcemap https://github.com/rolldown/rolldown/pull/7583#discussion_r2637206125
- [x] Warning for duplicate shebang https://github.com/rolldown/rolldown/pull/7583#issuecomment-3674790202