Fix gemspec_test to use Gem::Package#contents

Gem::Package#each isn't a public iteration API in modern rubygems;
#contents returns the full list of relative paths in the .gem and
matches what we need for the allowlist assertion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: cirdes

gem/test/gemspec_test.rb

@@ -43,7 +43,7 @@ def build_and_list_files
         output = `gem build ruby_ui.gemspec --output #{gem_path} 2>&1`
         raise "gem build failed:\n#{output}" unless $?.success?
       end
-      Gem::Package.new(gem_path).map { |entry| entry.full_name }
+      Gem::Package.new(gem_path).contents
     end
   end
 end