buffer: prevent string write offset overflow

[mcollina]

Reject offsets outside the destination buffer before subtracting from its length in SlowWriteString. Normalize wrapper arguments once so validated values reach the native binding.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.30%. Comparing base (f43086d) to head (f6f61c3).
⚠️ Report is 64 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/buffer.js	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65043      +/-   ##
==========================================
+ Coverage   90.27%   90.30%   +0.02%     
==========================================
  Files         762      759       -3     
  Lines      247515   247609      +94     
  Branches    46683    46683              
==========================================
+ Hits       223447   223603     +156     
+ Misses      15485    15460      -25     
+ Partials     8583     8546      -37     

Files with missing lines	Coverage Δ
src/node_buffer.cc	69.25% <ø> (-0.16%)	⬇️
lib/internal/buffer.js	98.93% <91.66%> (+0.18%)	⬆️

... and 60 files with indirect coverage changes

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