buffer: prevent abort on indexOf with lone surrogate needle

[RafaelGSS]

Buffer.prototype.indexOf(), lastIndexOf(), and includes() could abort when their UTF-8 string argument contained an unpaired UTF-16 surrogate. Size calculation now uses replacement-character semantics, matching Buffer.from() and the subsequent encoding step.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.12%. Comparing base (e7904d4) to head (923b472).
⚠️ Report is 353 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65430      +/-   ##
==========================================
- Coverage   90.14%   90.12%   -0.03%     
==========================================
  Files         746      752       +6     
  Lines      242764   252323    +9559     
  Branches    45739    47456    +1717     
==========================================
+ Hits       218848   227400    +8552     
- Misses      15426    16218     +792     
- Partials     8490     8705     +215     

Files with missing lines	Coverage Δ
src/string_bytes.cc	75.57% <100.00%> (+1.84%)	⬆️

... and 252 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/76069/