fix: use backend node id for file inputs

[oiahoon]

Summary

• identify file inputs with their stable backendNodeId when calling DOM.setFileInputFiles
• cover both the exact CDP command and a real Chrome file selection

Fixes #568

Verification

• bundle exec rspec spec/node_spec.rb --example '#select_file' (2 examples)
• bundle exec rake test (556 examples; the two local Chrome/path failures reproduce on unchanged main)
• bundle exec rubocop --parallel (99 files)
• bundle exec rake build
• git diff --check