sendImipInvitation() was fully implemented but never called after
createEvent or updateEvent — only sendImipCancellation was wired up
(in deleteEvent). This meant that even when the "send invitation"
checkbox was checked and participants were correctly saved on the
server, no invitation email was dispatched to attendees.
Apply the same pattern already used by deleteEvent: after a successful
create/update, if sendSchedulingMessages is true and the event has
participants, call sendImipInvitation() in a best-effort try/catch so
that email failures do not roll back the calendar operation.
For createEvent, the raw server response (created) is used directly
since it is already available and matches the CalendarEvent type
expected by sendImipInvitation.
For updateEvent, the updated event is reconstructed by merging the
existing store event with the incoming patch, avoiding an extra API
round-trip.
