fix(router): close multipart heartbeat connections after subscription closes

[df-wg]

Motivation and Context

Brings in the fix here: wundergraph/graphql-go-tools#1033

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.

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-1c11cb304195e6a8bd2504f184967c9eb1c680d6

[df-wg]

closed, merged in with #1532