Wikimedia providers don't provide images

[notapirate]

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• This is not a feature request
• This is not an image-provider request
• I have searched the issuetracker if the issue is not already reported.

Describe the bug:
The Wikimedia providers don't provide an image anymore. Root cause seem to be a missing user agent header in the image request, which Wikimedia requires for all requests, see the robot policy at https://w.wiki/4wJS, and further https://foundation.wikimedia.org/wiki/Policy:Wikimedia_Foundation_User-Agent_Policy.
This policy exists since 2010, but the bug only appeared recently, so I assume there was a change in curl behavior with PHP 8, maybe the user agent was sent by default before and is now empty.

To Reproduce:
Steps to reproduce the behavior:

1. Set a Wikimedia provider in the Design settings

Expected behavior:
The image shows in the preview and as background after logoff

Server:

• Nextcloud version: 31.0.8
• Splash-App version: 3.1.0

Additional Information:
One can simulate the request with curl, e.g.: curl -H 'User-Agent:' "https://commons.wikimedia.org/w/api.php?action=query&generator=images&titles=Template:Potd/2025-09-03&prop=imageinfo&iiprop=url&format=json"

[hiSandog]

For Wikimedia providers, the useful check is whether the provider returns no search results, bad image URLs, or metadata that the app rejects later. A small provider contract test could query one known Commons image/category and assert the normalized result includes a thumbnail URL, full image URL, author/license, and dimensions. That would make it clearer whether the breakage is API query syntax, response parsing, or license/size filtering.