Guests with a non-ASCII domain get the punycode address as display name #3929

Describe the bug

A guest invited with a non-ASCII domain gets the punycode address as display name, so share lists show x@xn--mller-kva.de instead of x@müller.de. Follow-up to the canonicalization from #3648.

Steps to reproduce

1. Share a folder with the guest address x@müller.de
2. Look at the share's collaborator (Graph grantedToV2.user.displayName)

Expected behavior

The display name is readable (x@müller.de); only the id/address stays canonical.

Actual behavior

displayName is the canonical punycode address (guestMailToIdentity in services/graph/pkg/service/v0/utils.go).

Setup

OpenCloud rolling (9.x), guest links enabled.

Additional context

Web now shows the readable form before sharing (opencloud-eu/web#4397).