Fix all golangci-lint issues across 35 files

[andrew]

Resolve 62 lint issues reported by gocritic, gocognit, gocyclo, dupl, mnd, unparam, ireturn, goconst, and errcheck linters. Net reduction of 819 lines.

• Extract test helpers to eliminate duplicate test code in carthage, cpan, gem, golang, maven, nuget, and pypi
• Reduce cognitive complexity by extracting helpers from parsers in rubygems, github_actions, golang, bun, npm, pnpm, and yarn
• Replace magic numbers with named constants across 15 packages
• Replace repeated string literals with constants in cpan, maven, pypi, nuget, and rubygems
• Simplify else-if chains in conan, golang, npm, and swift tests
• Remove unused function parameters in git and golang test helpers