[Bug]: Events created through `ICalendarEventBuilder` do not trigger invitation emails

[st3iny]

⚠️ 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

The embedded DAV server instantiated by the builder (through ICreateFromString) is not loading our IMipPlugin. Hence, no invitations will be sent.

Internally, a InvitationResponseServer is instantiated which does not include all plugins which a real CalDAV server would. I consider this to be a bug.

Ref

server/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php

Lines 82 to 86 in 64863c9

	$this->server->addPlugin(new \OCA\DAV\CalDAV\Plugin());
	$this->server->addPlugin(new \Sabre\CalDAV\ICSExportPlugin());
	$this->server->addPlugin(new \OCA\DAV\CalDAV\Schedule\Plugin(Server::get(IConfig::class), Server::get(LoggerInterface::class), Server::get(DefaultCalendarValidator::class)));
	$this->server->addPlugin(new \Sabre\CalDAV\Subscriptions\Plugin());
	$this->server->addPlugin(new \Sabre\CalDAV\Notifications\Plugin());

Proposed remediation:

1. Fix the interface to include more appropriate CalDAV plugins in the server.
2. Add a new method to ICreateFromString: createFromStringMinimal. This method keeps the old behavior as there are apps which actually rely on the "broken" logic, for example, Nextcloud Calendar (appointments).

Steps to reproduce

1. Build an event using the event builder with at least one invitee.
2. Create it in a calendar (ICreateFromString).

No invitation emails are dispatched.

Expected behavior

An invitation should be dispatched to each attendee.

Nextcloud Server version

31, main