src: add more checks and clarify docs for external references

[joyeecheung]

To help catch unregistered bindings.
Refs: #61718

[nodejs-github-bot]

Review requested:

• @nodejs/startup

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71231/

[codecov]

Codecov Report

❌ Patch coverage is 48.14815% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.71%. Comparing base (8e41b8d) to head (f4efb5f).
⚠️ Report is 42 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_snapshotable.cc	48.14%	11 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61719      +/-   ##
==========================================
- Coverage   89.74%   89.71%   -0.03%     
==========================================
  Files         674      675       +1     
  Lines      204348   204529     +181     
  Branches    39271    39309      +38     
==========================================
+ Hits       183396   183503     +107     
- Misses      13262    13295      +33     
- Partials     7690     7731      +41     

Files with missing lines	Coverage Δ
src/node_snapshotable.cc	72.94% <48.14%> (-0.71%)	⬇️

... and 58 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.