test(fetch): add userinfo coverage for issue-4897 URLs

[mcollina]

Summary

• add userinfo coverage ('' and user:pass@) to test/fetch/issue-4897.js as requested by @domenic
• keep path extraction assertions for non-credentialed URLs
• assert credentialed fetch URLs are rejected before dispatch with a TypeError mentioning credentials

Why

Follow-up to review nits on #4892 requesting extra coverage for URLs that include userinfo.

Testing

• npx borp -p "test/fetch/issue-4897.js"
• npx borp -p "test/websocket/issue-4889.js"

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.89%. Comparing base (4991f3e) to head (abeb7a0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4901      +/-   ##
==========================================
- Coverage   92.90%   92.89%   -0.01%     
==========================================
  Files         112      112              
  Lines       35638    35638              
==========================================
- Hits        33108    33106       -2     
- Misses       2530     2532       +2     

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