Fix expected status for metadata endpoint at npm group level

What does this MR do and why?

Changes the expected status to 404 Not found for unauthorized metadata requests to non public groups when request forwarding enabled.

Screenshots or screen recordings

Only test changes.

How to set up and validate locally

The following test should pass without any errors:

bundle exec rspec './spec/requests/api/npm_group_packages_spec.rb[1:1:1:1:1:15:1:1]'

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.