Use a single StringDecoder across reads so multi-byte UTF-8 sequences
that straddle the 8KB chunk boundary buffer correctly instead of
producing U+FFFD replacement characters that break JSON.parse.
Also fix the test mock: makeFakeFileHandle now advances an internal
cursor and returns bytesRead: 0 at EOF. The prior mock copied from
offset 0 every call, which would infinite-loop readJsonlPrefixLines
for any line larger than the 8192-byte chunk size.
Add a regression test using 3,000 CJK characters (9,000 bytes of
payload) to exercise the chunk boundary path.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
64b6a2ec72