lib: use bracket notation instead of startsWith/endsWith for single char

[kimtaejin3]

This PR avoids potential prototype pollution and improves performance by replacing method calls with
direct property access.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.12%. Comparing base (9472421) to head (5355e9f).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61500      +/-   ##
==========================================
- Coverage   90.12%   90.12%   -0.01%     
==========================================
  Files         752      752              
  Lines      252312   252319       +7     
  Branches    47432    47436       +4     
==========================================
+ Hits       227404   227406       +2     
  Misses      16217    16217              
- Partials     8691     8696       +5     

Files with missing lines	Coverage Δ
lib/internal/socketaddress.js	98.98% <100.00%> (ø)

... and 31 files with indirect coverage changes

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[kimtaejin3]

@anonrig Could you please check the progress of this PR? It looks like it might be stuck after approval.

[github-actions]

This pull request has been marked as stale due to 90 days of inactivity.
It will be automatically closed in 30 days if no further activity occurs. If this is still relevant, please leave a comment or update it to keep it open.

[trivikr]

@kimtaejin3 Can you please sign the first commit and force push?

It's required as per updated instructions

Your commit must contain the Signed-off-by line with your name and email address as an acknowledgement that you agree to the Developer Certificate of Origin