fix(subscription): never try to send on blocked channel when subscription was completed

[StarpTech]

Motivation and Context

Engine: wundergraph/graphql-go-tools#1100

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-652d24f3f607ae7cc4cd408fab2e21ab0f313dcc

[endigma]

Failing integration_test FYI, because its not required yet, looks like a race somewhere in kafka test hot path

[endigma]

there is a flaky data race in integration_test, requesting changes in-lieu of this workflow being required