chore: Gracefully shutdown the embedder

[Dschoordsch]

Description

Fixes #9693
End all streams on receiving a kill signal to properly shutdown the embedder. Ending just the merged stream does not work in the current situation as it will wait indefinitely if all the workers are idle.

Demo

Nothing to see here.

Testing scenarios

• run the embedder outside of pm2

  sh -c 'export SERVER_ID=7 && exec node dev/embedder.js'
  

• send SIGINT via Ctrl+C
• see the process ending gracefully

Final checklist

• I checked the code review guidelines
• I have added Metrics Representative as reviewer(s) if my PR invovles metrics/data/analytics related changes
• I have performed a self-review of my code, the same way I'd do it for any other team member
• I have tested all cases I listed in the testing scenarios and I haven't found any issues or regressions
• Whenever I took a non-obvious choice I added a comment explaining why I did it this way
• I added the label Skip Maintainer Review Indicating the PR only requires reviewer review and can be merged right after it's approved if the PR introduces only minor changes, does not contain any architectural changes or does not introduce any new patterns and I think one review is sufficient'
• PR title is human readable and could be used in changelog

[Dschoordsch]

This refactoring was done so we can define this return function. This is necessary so all streams are notified of the return. The async generator function we had previously would hang in await capability.promise and handle the return only afterwards. If all embedders are idle, the promise would never resolve.

[mattkrick]

interesting! really good find having to define our own return here

[Dschoordsch]

This was an attempt to regularly end the event loop. It will continue if there are still timers running unless these are unrefed. This did not allow proper shutdown as there are many things in dependencies still running, but it's good practice nonetheless if we're not keeping a reference to the timer to allow cleanup.