- POSIX: validate StringToSizeT result and token count when splitting
  the socket message into argv and additionalData; previously a
  malformed message could produce incorrect slicing.
- Windows: base64-encode additionalData before embedding in the
  null-delimited wchar_t buffer. The prior reinterpret_cast approach
  dropped everything after the first aligned 0x0000 in the serialized
  payload, so complex objects could arrive truncated.

Manually backported from #50119