fix(scripts): encode residual JSON control chars as \uXXXX instead of stripping (#1872)

* fix(scripts): encode residual control chars as \uXXXX instead of stripping

json_escape() was silently deleting control characters (U+0000-U+001F)
that were not individually handled (\n, \t, \r, \b, \f). Per RFC 8259,
these must be encoded as \uXXXX sequences to preserve data integrity.

Replace the tr -d strip with a char-by-char loop that emits proper
\uXXXX escapes for any remaining control characters.

* fix(scripts): address Copilot review on json_escape control char loop

- Set LC_ALL=C for the entire loop (not just printf) so that ${#s} and
  ${s:$i:1} operate on bytes deterministically across locales
- Fix comment: U+0000 (NUL) cannot exist in bash strings, range is
  U+0001-U+001F; adjust code guard accordingly (code >= 1)
- Emit directly to stdout instead of accumulating in a variable,
  avoiding quadratic string concatenation on longer inputs

* perf(scripts): use printf -v to avoid subshell in json_escape loop

Replace code=$(printf ...) with printf -v code to assign the character
code without spawning a subshell on every byte, reducing overhead for
longer inputs.