util: fix TextEncoder.encodeInto underfilling

[XadillaX]

TextEncoder.encodeInto() could underfill narrow destination buffers on the optimized path because:

• UTF-16 code units below U+0800 were incorrectly treated as three-byte UTF-8 sequences starting at U+0400.
• Latin-1 input was exposed as signed char while calculating the scalar tail.
• Surrogate pairs in the scalar tail were adjusted only after sizing, which could leave usable destination space unfilled.

Keep one-byte input unsigned, use the correct UTF-8 boundary, and account for surrogate pairs atomically. Use replacement-aware UTF-16 sizing and validate during conversion, avoiding a separate validation pass for valid input. A TODO records the preferred future simdutf bounded-conversion API that would return both consumed and written counts.

Fixes: #65994

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.03%. Comparing base (565f69f) to head (fe02f30).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/encoding_binding.cc	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65997      +/-   ##
==========================================
+ Coverage   89.99%   90.03%   +0.03%     
==========================================
  Files         784      784              
  Lines      268410   268411       +1     
  Branches    51124    51125       +1     
==========================================
+ Hits       241562   241651      +89     
+ Misses      17385    17291      -94     
- Partials     9463     9469       +6     

Files with missing lines	Coverage Δ
src/encoding_binding.cc	87.69% <93.93%> (+26.58%)	⬆️

... and 31 files with indirect coverage changes

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