[Bug]: Delegated calendar home returns 404 when an app-generated Deck calendar is missing

Bug description

After configuring a read-write calendar delegate, the delegate cannot load the delegated calendars in the Calendar web app.

The Calendar app shows:

Could not load delegated calendars. Make sure the server supports calendar delegation.

The delegation itself is stored correctly:

owner_id=principals/users/agape
proxy_id=principals/users/AlexeyRu
permissions=3

The delegated user's principal is discovered successfully, but loading the delegated calendar home fails:

PROPFIND /remote.php/dav/principals/users/agape 207
PROPFIND /remote.php/dav/calendars/agape/ 404

The Nextcloud log for the failed calendar-home request contains:

Sabre\DAV\Exception\NotFound
Node with name 'app-generated--deck--board-1' could not be found

This appears related to #42051, but the missing node is an app-generated Deck calendar rather than trashbin. The fixes from #60648 / stable34 backport #60968 are already present in the installed server code.

Steps to reproduce

1. Use an account that has calendars and enable Deck calendar integration.
2. As calendar owner agape, add another local user as a read-write delegate.
3. Confirm the proxy relation was stored with permission 3.
4. Sign in as the delegate.
5. Open the Calendar app.

Expected behavior

The delegated calendar home loads and the owner's calendars are available to the delegate.

Actual behavior

The Calendar app shows the delegated-calendar loading error. The delegated calendar-home PROPFIND returns 404 because an app-generated Deck calendar node cannot be found.

Server configuration

• Nextcloud Server: 34.0.1.2
• Calendar: 6.5.0
• Deck: 1.18.2
• PHP: Docker FPM image based on nextcloud:34.0.1-fpm
• Database: MariaDB 11
• Browser: Chrome 150 on Windows 10
• Deployment: Docker Compose behind nginx

Additional context

Normal calendars of the delegate load successfully. The failure occurs specifically while discovering the delegated owner's calendar home. Comparing the installed DAV files with the current stable34 branch showed no relevant difference, so there does not appear to be a newer stable34 fix at the time of reporting.

Start by reproducing the delegated calendar-home PROPFIND against /remote.php/dav/calendars/agape/ and trace handling of the missing app-generated--deck--board-1 node, using the related issues #42051, #60648, and #60968 as context. Done means the delegated calendar home loads successfully and the owner's calendars are available to the delegate without a 404.