fix: Public link with multiple mounts

[SystemKeeper]

Summary

When multiple mounts with different permissions are available to access a file, a public link for such a file might access the wrong (non-shareable) node and show a 404 instead of the file.
Question generally would be if it makes sense to extract that code (e.g. getFirstShareableNodeById or a separate class to resolve these types of nodes).

The test was created with help of AI, which pointed out that a similar exists at

server/apps/files_sharing/lib/Listener/RestrictInteractionListener.php

Lines 54 to 56 in 4fcbb98

	if (!$resource->getNode()->isShareable()) {
	throw new InteractionRestrictedException('Node is not shareable.', $this->l10n->t('You are not allowed to share "%s".', [$resource->getNode()->getName()]));
	}

and the check should probably use getNodePermissions there. This seems 34+ only, the fix here is also < 34.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[SystemKeeper]

/backport to stable34

[SystemKeeper]

/backport to stable33

[SystemKeeper]

/backport to stable32