ref(nestjs): Move event instrumentation unit tests to separate file

[github-actions]

Note

The pull request "ref(nestjs): Move event instrumentation unit tests to separate file" was created by @nicohrubec but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

No changes just moving the tests to a separate file to make it a bit more explicit what is what. Also updated the comment at the top of the event instrumentation to represent the updated behavior.

[linear-code]

JS-1913

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #19738, which was included in the 10.44.0 release.