On Windows, std::tmpfile() returns NULL with errno 22 when %TEMP%
contains a character outside the ANSI code page, where GetTempPathA
returns '?' in its place. Fifteen tests that capture output through it
aborted. TemporaryFile::Open() opens a unique delete-on-close file under
temp_directory_path() with _wfopen; other hosts keep std::tmpfile().
Part of #13083
Co-Authored-By: Claude Opus 5.5 <[email protected]>
