Not possible to set a guest name in public share

[mejo-]

Describe the bug
In a publically shared document, we allow users to set a guest name during the session. This functionality is broken in Nextcloud 26 and 27.

From a first glance it looks like it didn't get migrated along with the recent syncService changes. @juliushaertl and @max-nextcloud since you have the most knowledge here, could you take a look?

To Reproduce
Steps to reproduce the behavior:

1. Create a public share link for a document (with editing permissions)
2. Open the public share link in a private tab, open the active users list
3. Try to set a name for your session user
4. See the error this.$syncService.session is undefined
5. If you mitigate this error, next one is this.$syncService.updateSession(...) is undefined

[siccovansas]

This bug still exists in Hub 6/Nextcloud 27.1.2