utils/cpan: add offline test coverage

[p-linnane]

utils/cpan.rb was almost entirely untested: the existing spec covered URL version parsing and valid_cpan_package?, and nothing else. Every path that talks to MetaCPAN or rewrites a formula had no coverage at all.

This adds offline coverage for CPAN::Package#latest_cpan_info (success, per-instance caching, non-CPAN short circuit, HTTP failure, malformed JSON, missing download_url, missing checksum_sha256) and for CPAN.update_perl_resources! (print-only output, in-place AST rewrite, version reporting, already-current resources, no CPAN resources, unresolvable metadata, and the --ignore-errors write path). Utils::Curl.curl_output is stubbed throughout so nothing hits the network, unlike the equivalent PyPI coverage in pypi_spec.rb which is gated behind :needs_network.

I want this in before touching utils/cpan.rb itself. Two production issues are already known: non-CPAN resources adjacent to CPAN ones are silently dropped during the resource-block rewrite, and a MetaCPAN response missing version produces an array containing nil despite the declared T::Array[String] return type. Both need behavior changes, and those are easier to review against a spec that already pins the current contract.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code (Opus 5) drafted the tests; I reviewed the diff, verified each new example fails against a mutated implementation and passes against the real one, and ran brew lgtm plus targeted specs.

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[MikeMcQuaid]

Thanks!

[Copilot]

Pull request overview

Adds comprehensive offline RSpec coverage for utils/cpan.rb, focusing on the previously untested MetaCPAN fetch path and the formula resource stanza rewrite/update logic, by stubbing Utils::Curl.curl_output to avoid any network access.

Changes:

• Add offline specs for CPAN::Package#latest_cpan_info, covering success, caching, and multiple failure/malformed-data cases.
• Add offline specs for CPAN.update_perl_resources!, covering print-only output, in-place formula rewrite, version/update reporting, and error/--ignore-errors behavior.

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