Add tool to verify YAMLSerializer can load gemspecs of popular gems and their dependencies

[hsbt]

What was the end-user or developer problem that led to this PR?

from #9352 (comment)

I've tested to load gemspec of popular gems and their dependencies with YAMLSerializer.

What is your fix for the problem, implemented in this PR?

This scripts download and load 773 gems from 300 top downloads. YAMLSerializer could load them.

You can test that with:

$ ruby --yjit -Ilib --disable-gems tool/test_yaml_serializer_with_popular_gems.rb 300

I'm not sure how much testing is needed before it becomes practical, but I want to continue testing.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[Copilot]

Pull request overview

Adds a new RubyGems tooling script to sanity-check that Gem::YAMLSerializer can parse real-world gemspec YAML by downloading gem files for a set of popular gems and their transitive runtime dependencies.

Changes:

• Introduce tool/test_yaml_serializer_with_popular_gems.rb to fetch top-N gems, resolve runtime dependency closure, download .gem files, and run Gem::YAMLSerializer.load on their gemspec metadata.

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[hsbt]

I verified Top 1000 download gems and 404 transitive dependencies. The only gem whose gemspec could not be parsed was the escape gem.

I fixed that at #9399

[hsbt]

After thinking about it overnight, this script doesn't need to be included in the repository. So I'm closing this.