Reject stub managed Node runtimes

[remybanks77]

Summary

First focused split from the larger closed #313 branch, following maintainer guidance to break the work into smaller PRs.

• Changes the managed Node runtime probe from node -v to a real node -e execution check.
• Rejects version-only stub binaries that print a version but cannot execute JavaScript.
• Adds smoke coverage proving an existing stub runtime is replaced by a real managed runtime source.

Validation

Local on 9e6ef10:

• bash -n tests/scripts_smoke.sh
• git diff --check origin/main...HEAD
• bash tests/scripts_smoke.sh -> all script smoke tests passed

GitHub on 9e6ef10:

• CI run 26493942582 passed.

Notes

This is intentionally narrow: scripts/lib/node-runtime.sh plus its smoke tests only.

[ilysenko]

Reviewed the current head and verified the managed Node runtime probe change plus smoke coverage. This is narrow and safe to merge.