Support YAML !binary tag in YAMLSerializer

[Copilot]

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

Gems packaged with older RubyGems encode checksums.yaml.gz values using YAML 1.1 !binary tags (base64). When Psych is disabled/unavailable, Gem::YAMLSerializer failed to parse these, causing gem install failures.

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

Extended Gem::YAMLSerializer::Parser to base64-decode scalars tagged with !binary in:

• Mapping keys and values (inline and block scalar)
• Sequence items (inline)

Also adds test coverage for the sequence item code path (- !binary "..."), which was missing from the initial implementation.

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

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Pull request overview

This PR improves RubyGems’ Psych-less YAML parsing path by teaching Gem::YAMLSerializer to handle YAML 1.1 !binary (base64) scalars, which appear in older gems’ checksums.yaml.gz and can otherwise break installs.

Changes:

• Add support in Gem::YAMLSerializer::Parser for base64-decoding !binary scalars across mappings (keys/values) and sequences.
• Add a regression test covering the inline sequence-item !binary case (- !binary "...").

Reviewed changes

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

File	Description
lib/rubygems/yaml_serializer.rb	Extends the YAMLSerializer parser to recognize and decode !binary-tagged scalars via base64.
test/rubygems/test_gem_safe_yaml.rb	Adds coverage for decoding !binary used as an inline sequence item.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.