[Bug]: Studio document preview fails when the page count is not known yet #623

Problem

In Pimcore Studio, the preview of a non-PDF document asset fails ("not found") whenever the asset has no page count yet.

DocumentService::validateAssetDocument() in studio-backend-bundle (2026.3, src/Asset/Service/DocumentService.php:133) throws ElementStreamResourceNotFoundException when $asset->getPageCount() is empty. getPageCount() reads the pageCount custom setting, which the asset update-tasks queue writes. It is null until that queue has processed the asset, and stays null permanently if page-count extraction failed or the setting was never written. The document itself would convert fine.

Expected

Let the PDF conversion decide. The conversion path already handles an unconvertible document, so the preview should only fail when the conversion actually fails.

Notes

• This is the Studio half of [Bug]: Document preview and thumbnail are suppressed when the page count is not known #382, which was closed because the admin-ui-classic part won't be fixed (admin-ui-classic is being sunset).
• The thumbnail half doesn't apply to Studio: DownloadService::streamDocumentThumbnailByName() never checks the page count.
• Internal reference: PEES-1553.