src: fix two encodeInto() bugs that reject input that fits

[agape1225]

TextEncoder.encodeInto() could report that a code point does not fit in the destination Uint8Array even when it does.

1. simpleUtfEncodingLength() used 0x400 as the boundary between 2-byte and 3-byte UTF-8 encodings, but the correct boundary is 0x800: code points in [0x80, 0x800) need 2 bytes in UTF-8, and only code points

   = 0x800 need 3.

2. The same function is called with a raw char from the Latin1 (one-byte string) code path. char is signed on some platforms, so a byte >= 0x80 gets sign-extended to a large uint16_t value instead of the intended code point, which also made encodeInto() behave differently for the same prefix depending on whether the rest of the source string forced V8 to represent it as one-byte (Latin1) or two-byte (UTF-16) internally.

Fixes: #65994

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.99%. Comparing base (565f69f) to head (4e69ac9).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/encoding_binding.cc	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65998      +/-   ##
==========================================
- Coverage   89.99%   89.99%   -0.01%     
==========================================
  Files         784      784              
  Lines      268410   268411       +1     
  Branches    51124    51131       +7     
==========================================
- Hits       241562   241561       -1     
+ Misses      17385    17366      -19     
- Partials     9463     9484      +21     

Files with missing lines	Coverage Δ
src/encoding_binding.cc	76.92% <66.66%> (+15.81%)	⬆️

... and 29 files with indirect coverage changes

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