"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

Start with the linked CodeSandbox reproduction and compare the embedded and non-embedded Sandbox behavior when the headers option is supplied. Confirm the fix by opening the Headers tab in the embedded Sandbox and verifying that the supplied headers appear there.

Written by the indexing model from the issue text.