reject unpaired high surrogate in wchar_t string formatting

[nabhan06]

The wide-string overload of ConvertStringArg in str_format runs each wchar_t through WideToUtf8 but never inspects the ShiftState after the loop, so a wide string ending on an unpaired UTF-16 high surrogate has only the first two bytes of a 4-byte sequence written while StrFormat("%ls", ...) still reports success, emitting truncated invalid UTF-8. The single-character path in ConvertWCharTImpl already rejects a lone high surrogate, so this adds the same saw_high_surrogate check after the loop to make the string path fail the same way. The regression test in convert_test.cc covers both a trailing high surrogate and a valid non-BMP code point so surrogate pairs keep working.