Support YAML !binary tag in YAMLSerializer

[hsbt]

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

Decode base64-encoded values tagged with !binary in mapping keys, mapping values (both inline and block scalar), and sequence items.

This fixes gem install failures for gems packaged with older RubyGems that used !binary encoding in checksums.yaml.gz.

Fixes #9387

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 support in RubyGems’ pure-Ruby Gem::YAMLSerializer to correctly decode YAML 1.1 !binary-tagged scalars (base64) so older gems’ checksums.yaml.gz can be parsed/verified when Psych is disabled/unavailable (fixing #9387).

Changes:

• Decode !binary values in Gem::YAMLSerializer::Parser for mapping keys and values, and for sequence items (including block scalars).
• Add regression tests covering !binary decoding in mapping keys and mapping values (inline + block scalar), including an “older checksums.yaml.gz” style fixture.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
test/rubygems/test_gem_safe_yaml.rb	Adds regression tests for !binary decoding scenarios (keys + values).
lib/rubygems/yaml_serializer.rb	Extends the YAMLSerializer parser to decode !binary-tagged scalars via base64 decoding.

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[Copilot]

The PR description mentions decoding !binary tags in sequence items, and the parser now has a parse_sequence_item branch for !binary, but there is no test that exercises a - !binary ... sequence item (inline and/or block scalar). Adding a dedicated test would prevent regressions for this code path.

Suggested change

	end
	end
	def test_binary_tag_decoded_in_sequence_item_inline
	yaml = <<~YAML
	---
	- !binary "U0hBMQ=="
	YAML
	result = yaml_load(yaml)
	assert_equal ["SHA1"], result
	end

[hsbt]

@copilot open a new pull request to apply changes based on the comments in this thread

[Copilot]

@hsbt I've opened a new pull request, #9393, to work on those changes. Once the pull request is ready, I'll request review from you.