fix: remove support for non-real Blob objects

[mcollina]

Description

• restrict isBlobLike() to real Blob instances
• stop accepting Blob/File lookalikes based on Symbol.toStringTag
• add regression coverage for fake blob-like values in util and request validation tests

Testing

• npx borp -p "test/util.js"
• node --test test/node-test/validations.js

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (4b71486) to head (a23078f).

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4937      +/-   ##
==========================================
- Coverage   92.94%   92.94%   -0.01%     
==========================================
  Files         111      111              
  Lines       35829    35815      -14     
==========================================
- Hits        33302    33287      -15     
- Misses       2527     2528       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 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.