chore: add test to verify behaviour of heartbeat with SSE subscriptions

[alepane21]

@coderabbitai summary

Checklist

• [ ] I have discussed my proposed changes in an issue and have received approval to proceed.
• [ ] I have followed the coding standards of the project.
• [ ] Tests or benchmarks have been added or updated.
• [ ] Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• [ ] I have read the Contributors Guide.

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[github-actions[bot]]

Router image scan passed

:white_check_mark: No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-489981b3f4f4a01be746295d8f29581feba9dc76

[github-actions[bot]]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions[bot]]

Closed as inactive. Feel free to reopen if this PR is still being worked on.