Tests BufferedJsonFormatter output: nesting and commas, number and bool
values, skipped null strings and escaping. A canary after the buffer
catches writes past its end.
Three fail: control characters other than \b\f\n\r\t aren't escaped,
calcEscapedLength skips the first character, and so a value starting
with an escaped character can be written one byte past the buffer.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
