fix: harden Ruby mapper edge cases

[steipete]

Summary

• harden Ruby/Rails mapper detection around generated gemspecs, nested gemspec scope, Ruby test naming, Rails config/assets/database slices, and source-vs-CLI scripts handling
• keep RuboCop extension gems as lint providers and preserve Ruby test associations for prefixed/suffix Minitest layouts
• document sensitive Rails config exclusions and thank @inertia186 in the changelog

Proof

• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm test src/mapper.test.ts
• pnpm test
• pnpm build
• codex-review --mode branch --parallel-tests "pnpm test src/mapper.test.ts"