Fix and dequarantine pipeline security E2E tests

What does this MR do and why?

The two pipeline security vulnerability tests were quarantined as flaky, but they were actually broken by UI changes in the security dashboard.

Fixes:

• Select a finding by its checkbox instead of the status text, which is no longer clickable.
• Filter by status using the current filtered-search flow (the status token is now a flat list on the pipeline tab).
• Verify a dismissed vulnerability using the filter_by_status helper so the new default "Still detected" activity filter no longer hides it.

Both tests now pass, so the quarantine tags are removed.

References

• Related to https://gitlab.com/gitlab-org/quality/test-failure-issues/-/issues/22185
• Epic: gitlab-org#22311

How to set up and validate locally

Run the two examples against a local GDK that has a project runner available:

1. cd qa

2. Run the examples directly:

   bundle install
   
   QA_GITLAB_URL=http://gdk.test:3000 WEBDRIVER_HEADLESS=true \
     bundle exec rspec \
     qa/specs/features/ee/browser_ui/18_security_risk_management/vulnerability_management_spec.rb:63 \
     qa/specs/features/ee/browser_ui/18_security_risk_management/vulnerability_management_spec.rb:85