Just to be clear here:
on 13.2 you are likely using kernel-desktop, so installing virtualbox-guest-kmp-default doesn’t make sense.
You need virtualbox-guest-kmp-desktop.
Despite what tsu2 wrote, I don’t think there’s a problem with the guest additions in 13.2 (and no, it is not considered EOL yet by far).
That’s only a problem in Tumbleweed currently, because of the newer kernel and Xorg (a fix is on the way though AIUI).
There is one “problem” though: the installation location of the guest kernel modules have been moved from /lib/modules/xxx/updates to /lib/modules/xxx/misc to be more aligned with the upstream guest additions.
Unfortunately, /lib/modules/xxx/updates is searched before misc, so if you still have an older version installed (because of the kernel multiversion feature), it won’t work because the kernel will load the older version.
uninstall virtualbox-guest-kmp-desktop in the guest completely, and reinstall the latest version:
sudo rpm -e --nodeps --allmatches virtualbox-guest-kmp-desktop
sudo zypper in virtualbox-guest-kmp-desktop