[Bug] (caldav): Adding attendee to existing event generates update message instead of new invitation

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When adding an attendee to a existing event with attendees the attendee gets an update message.

Steps to reproduce

1. Create event with 1 attendee
2. Edit the event and 2nd attendee
3. Check email for 2nd attendee

Expected behavior

The new attendee should get a new invite and NOT a update message

Installation method

None

Nextcloud Server version

master

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.3

Web server

Nginx

Database engine version

MySQL

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

Cause:
This issue is caused by the OCA\DAV\CalDAV\Schedule\IMipPlugin::schedule() function due to insufficient information provided from the iTipMessage generated by the Sabre\VObject\ITip\Broker::parseEventForOrganizer() function.

Solution:
Update up stream repo to include more information in iTipMessage

Start with OCA\DAV\CalDAV\Schedule\IMipPlugin::schedule() and inspect the iTipMessage produced by Sabre\VObject\ITip\Broker::parseEventForOrganizer(). Reproduce the event-edit flow with an existing attendee and a newly added attendee, then trace which information is missing. Done means the new attendee receives a new invitation rather than an update message, with the required upstream change included.