Use JSON for cargo metadata parsing

[hsbt]

from #9352

I'm working to replace Psych to pure Ruby implementation now. It's hard to support JSON at YAMLSerializer.

It's no problem to load json at RubyGems because lib/rubygems/ext/cargo_builder.rb is only called at gem i.

[Copilot]

Pull request overview

This PR switches Rust extension installation (Cargo metadata parsing) from YAML-based parsing to JSON parsing, aligning with the effort to reduce reliance on Psych/YAML internals within RubyGems.

Changes:

• Replace Gem::SafeYAML.safe_load with JSON.parse for cargo metadata output.
• Add a local require "json" to support JSON parsing in the cargo builder.

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[Copilot]

Open3.capture2e merges stdout+stderr, but cargo metadata writes JSON to stdout and can emit warnings/diagnostics to stderr even on success. With the new JSON.parse(output), any stderr content will make the string invalid JSON and break installs. Capture stdout separately (e.g., Open3.capture3 or capture2 with stderr captured independently) and parse only stdout; keep stderr for really_verbose/results output.

[Copilot]

Now that cargo metadata is parsed as JSON, the earlier Gem.load_yaml in this method is unused and forces the YAML engine to be loaded during Rust extension installs. Please remove that call to avoid an unnecessary dependency/load cost (and to align with the goal of moving away from Psych).

[Copilot]

If JSON.parse fails (e.g., unexpected cargo output), it will currently raise JSON::ParserError and may surface as a noisy backtrace during gem install. Consider rescuing parse errors here and re-raising Gem::InstallError with a concise message (and include the raw output only when really_verbose or via results).

Suggested change

	metadata = JSON.parse(output)
	metadata =
	begin
	JSON.parse(output)
	rescue JSON::ParserError => error
	if Gem.configuration.really_verbose
	puts output
	else
	results << output
	end
	raise Gem::InstallError, "cargo metadata produced invalid JSON: #{error.message}"
	end