"headers" not respected for embedded Sandbox
### Issue Description

When passing the [`headers` option](https://www.apollographql.com/docs/apollo-server/api/plugin/landing-pages/#headers) to an embedded Sandbox, the supplied headers are not set in the UI. Potentially related to #7685.

### Link to Reproduction

https://codesandbox.io/p/devbox/nostalgic-chebyshev-shhcqq

### Reproduction Steps

- Run `yarn start`
- In the embedded sandbox, open the "Headers" tab and see the headers have not been set
- In the options, set `embed: false`
- In the linked sandbox, open the "Headers" tab and see the headers have been set