src: remove double encoding validation on fatal path

[araujogui]

Create StringBytes::EncodeValidatedUTF8 to avoid double UTF-8 validation on fatal path

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR optimizes UTF-8 decoding in the encoding binding by avoiding redundant UTF-8 validation when input has already been validated.

Changes:

• Added StringBytes::EncodeValidatedUTF8() API for encoding already-validated UTF-8.
• Refactored the fast UTF-8→UTF-16 conversion into a shared helper (EncodeKnownValidNonAsciiUTF8).
• Updated BindingData::DecodeUTF8() to use the new API after performing UTF-8 validation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/string_bytes.h	Declares a new EncodeValidatedUTF8() entry point for already-validated UTF-8.
src/string_bytes.cc	Implements EncodeValidatedUTF8() and factors out the fast conversion helper used by both code paths.
src/encoding_binding.cc	Uses EncodeValidatedUTF8() after explicit UTF-8 validation to avoid re-validation overhead.

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[codecov]

Codecov Report

❌ Patch coverage is 68.96552% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.31%. Comparing base (49dcaa0) to head (727b683).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/string_bytes.cc	65.21%	4 Missing and 4 partials ⚠️
src/encoding_binding.cc	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63587      +/-   ##
==========================================
- Coverage   90.33%   90.31%   -0.03%     
==========================================
  Files         730      730              
  Lines      234362   234671     +309     
  Branches    43906    43946      +40     
==========================================
+ Hits       211713   211941     +228     
- Misses      14371    14450      +79     
- Partials     8278     8280       +2     

Files with missing lines	Coverage Δ
src/string_bytes.h	80.00% <ø> (ø)
src/encoding_binding.cc	53.35% <83.33%> (+0.43%)	⬆️
src/string_bytes.cc	74.43% <65.21%> (-0.20%)	⬇️

... and 49 files with indirect coverage changes

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