test: add parent upgrade remediation tests

[sonukapoor]

Summary

• add unit tests for src/remediation/parent-upgrade.ts
• cover non-transitive skips, missing direct parents, exact direct-child upgrade recommendations, deeper best-effort recommendations, and failed packument fetches
• isolate module-level packument caching in tests so each remediation scenario is validated independently

Testing

• npm test
• npm run build

Closes #61