[Bug] Asset folder preview: trigger generation of preview on request

[brusch]

… fixes #1096

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

Fixes missing async generation trigger for asset folder content previews by ensuring preview generation is dispatched when a tree-preview thumbnail is requested but not yet available.

Changes:

• Always dispatch AssetPreviewImageMessage when treepreview is requested and the preview thumbnail does not exist (not just for origin=treeNode).
• Keep existing origin-specific responses (404 for treeNode, loading GIF for folderPreview) while triggering generation.

Comments suppressed due to low confidence (1)

src/Controller/Admin/Asset/AssetController.php:1181

• getFolderContentPreviewAction() builds preview URLs for all non-folder assets with origin=folderPreview (via ElementService::getThumbnailUrl()), but only the image thumbnail endpoint dispatches AssetPreviewImageMessage for folderPreview. The video and document thumbnail endpoints still dispatch only for origin=treeNode, so folderPreview requests won’t enqueue preview generation when those thumbnails are missing. Consider aligning getVideoThumbnailAction() / getDocumentThumbnailAction() with the image behavior (dispatch + placeholder/appropriate response) or otherwise ensuring folder previews can self-trigger generation for those asset types too.

                \Pimcore::getContainer()->get('messenger.bus.pimcore-core')->dispatch(
                    new AssetPreviewImageMessage($image->getId())
                );

                if ($request->get('origin') === 'treeNode') {
                    throw $this->createNotFoundException(sprintf('Tree preview thumbnail not available for asset %s', $image->getId()));
                } elseif ($request->get('origin') === 'folderPreview') {
                    return new BinaryFileResponse(PIMCORE_WEB_ROOT . '/bundles/pimcoreadmin/img/video-loading.gif');
                }

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