validate utf-8 continuation bytes in utf8ToCodepoint

[SABITHSAHEB]

1. utf8ToCodepoint consumed the bytes after a multibyte lead without checking they are continuation bytes (10xxxxxx), so a broken sequence swallowed the ASCII that followed it. With the default emitUTF8=off, serializing a string containing "\xE0AB" produced "�" and dropped the valid A and B; "\xF0XYZ" produced a bogus surrogate pair with X, Y, Z gone.
2. a well-formed 4-byte sequence decoding past U+10FFFF was still emitted, as an invalid surrogate pair.
   Validate each trailing byte and cap at U+10FFFF, returning U+FFFD without advancing past bytes that are not part of the sequence. This is the lenient replacement behaviour rather than a hard error, so valid UTF-8 output is unchanged.

[SABITHSAHEB]

any update?

[baylesj]

@greptileai

[greptile-apps]

Greptile Summary

The PR validates UTF-8 continuation bytes during JSON string serialization and rejects four-byte sequences above U+10FFFF.

• Preserves valid bytes following malformed multibyte prefixes.
• Consumes valid continuation prefixes as part of a single replacement character.
• Adds regression coverage for malformed and out-of-range sequences.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the previously reported continuation-prefix issue is fixed by consuming the valid prefix once while preserving the offending byte for the caller’s next iteration.

Important Files Changed

Filename	Overview
src/lib_json/json_writer.cpp	Validates continuation bytes, preserves the first offending byte for subsequent decoding, and rejects code points above U+10FFFF.
src/test_lib_json/main.cpp	Adds regression tests covering malformed prefixes, preserved ASCII suffixes, out-of-range code points, and valid multibyte input.

_{Reviews (2): Last reviewed commit: "consume the valid continuation prefix of..." | Re-trigger Greptile}

[coveralls]

Coverage Report for CI Build 32591467250

Coverage increased (+0.03%) to 89.935%

Details

• Coverage increased (+0.03%) from the base build.
• Patch coverage: 6 of 6 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	2745
Covered Lines:	2620
Line Coverage:	95.45%
Relevant Branches:	2650
Covered Branches:	2232
Branch Coverage:	84.23%
Branches in Coverage %:	Yes
Coverage Strength:	23787.12 hits per line

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💛 - Coveralls

[SABITHSAHEB]

@baylesj addressed the one item from the review above (valid continuation prefix was being replaced byte by byte). The truncated-at-end-of-input case goes through the pre-existing length checks, so I left that path alone.