fix(previews): ffprobe hangs

[adduxa]

• Resolves: [Bug]: preview:generate-all hangs indefinitely on a specific .MTS video file waiting for ffprobe #60915

Summary

Fix pipe buffer deadlock in useHdr() in lib/private/Preview/Movie.php.

When ffprobe produces more than ~64KB on stderr, the sequential stream_get_contents calls
deadlock: PHP blocks reading stdout while ffprobe blocks flushing stderr. This caused
occ preview:generate-all to hang indefinitely on certain video files.

A tempting workaround is to swap the two stream_get_contents calls (reading stderr first),
but I'm not sure if ffprobe is guaranteed to produce <64KB on the stdout for this call.

Fix drains both pipes concurrently using stream_select with non-blocking I/O.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI