fix(events): a meeting moved to a new host is sent with the attendees as given (PPT-2640)

[camreeves]

What

When a host change moves a meeting to the new host's calendar (#383), update added the previous host back to the attendees before the meeting was re-sent. So the previous host stayed in the moved meeting even when the edit removed them, which is what QA hit on PPT-2640: Lynne removed from the attendees, request correct, response and calendar still have Lynne.

The forced host attendee is now skipped when the meeting is moving to a new host, so the moved meeting is sent with the attendees as given. The frontend keeps the previous host in the list by default (the "previous host will remain an attendee" warning), so the default flow is unchanged and an explicit removal now sticks. Edits without a host change are unchanged.

Testing

• The existing transfer spec asserted the previous host is always invited, with an input that never listed them. It now asserts they are not invited when the request leaves them out, and a new case covers them being kept when the request still lists them.
• spec/controllers/events_spec.cr: 57 examples, 0 failures. The same spec against master fails on the new assertion.
• Tested on placeos-dev against Office365 with this branch deployed, signed in as a sandbox user, using the WorkMate edit form:
  • host changed Lynne -> Pradeep and Lynne removed from the attendees: the meeting moved to Pradeep's calendar with Megan, the room and Pradeep, and it is no longer in Lynne's calendar.
  • host changed Lynne -> Pradeep with Lynne left in the attendees: the moved meeting still invites her and it lands in her calendar as an attendee.
• Separate finding, not changed here: on a delegated tenant the move only works when the person editing can write to the new host's calendar. Where they can't (seen on HIO UAT), Graph returns ErrorItemNotFound and the meeting is restored on the previous host.

PPT-2640