Normalize internal Next.js loader paths in redbox source snapshots
Add pattern to match next/dist/build/webpack/loaders/ paths that appear
in both CI (./node_modules/next/dist/...) and local (../packages/next/dist/...)
environments, replacing them with <FIXME-nextjs-internal-source>.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>