Public link root reports public-link-item-type "folder" for links to a single file #3693

Describe the bug

For a public link to a single file, the root of the public link reports oc:public-link-item-type = folder (the virtual folder that contains the file). Folder links report folder as well, so clients can't tell from the link root whether a link points to a file or a folder.

Web needs that to hide actions that are a dead end on single-file links, e.g. "Insert image from cloud" in the text editor: the file picker can only show the shared file itself there (opencloud-eu/web#3631, opencloud-eu/web#3632). For now, web hides the action on all public links, also on folder links where picking an image from the shared folder would be useful.

Steps to reproduce

1. Create a public link for a single file, e.g. form.pdf (here with a password).
2. Request the link root:

   curl -u "public:<password>" -X PROPFIND -H "Depth: 1" \
     --data '<?xml version="1.0"?><d:propfind xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns"><d:prop><oc:public-link-item-type/><d:resourcetype/></d:prop></d:propfind>' \
     https://<host>/remote.php/dav/public-files/<token>

3. Compare with a public link to a folder.

Expected behavior

The link root reports the type of the shared item: file for a link to a single file, folder for a link to a folder (the entries inside keep reporting their own type).

Actual behavior

Both report folder for the root:

(For OCM single-file shares, web already handles the root being reported as file, see packages/web-client/src/webdav/listFiles.ts.)

Setup

opencloudeu/opencloud-rolling:daily, product version 8.1.0+fbb0aff1, started with the docker compose setup of the web repository (docker-compose.yml).

Additional context

If reporting file for the root isn't desired, another property of the link root that tells the shared item's type would work for web as well.