Dismiss the iOS native editor when the contact is deleted from it

[liliana-tagonsoft]

Description

On iOS, FlutterContacts.native.showEditor(contactId) presents a CNContactViewController with
allowsEditing = true. Tapping the editor's Delete Contact row deletes the contact, but:

• contactViewController(_:didCompleteWith:) is never called, so the sheet stays on screen with a
  now-deleted contact still rendered in it;
• the showEditor Future doesn't complete until the user closes the sheet by hand — the controller
  is presented in editing mode, so it shows its own ✕ / ✓ items rather than the injected Done
  button — and it then resolves to null, indistinguishable from "cancelled", so the app has no way
  to know the contact was deleted;
• calling showEditor again for the same id then throws
  PlatformException(flutter_contacts_error, Updated Record Does Not Exist), because
  store.unifiedContact(withIdentifier:) fails on the deleted identifier.

Note the Delete Contact row only appears in a third-party CNContactViewController on iOS 26+
(it isn't offered on earlier versions), which is probably why this hasn't come up before.

Fix

While the editor is presented, observe .CNContactStoreDidChange. When a store change arrives and
unifiedContact(withIdentifier:) no longer returns the edited contact, dismiss the editor and
complete the pending result with nil. The dismissal goes through
navController.presentingViewController so the delete confirmation sheet, if it's still animating
out, goes away with the editor. The observer is removed in completeWithResult, so it's torn down
on every exit path (delegate, close, deletion).

Saves are unaffected: the notification also fires on a normal edit, but the contact still resolves,
so the observer returns early and the existing delegate path handles it.

Testing

Tested in a production app on the iOS 26.3 simulator (iPhone 17 Pro):

• Editor → Delete Contact → confirm: the sheet now dismisses itself and showEditor completes
  with null.
• Editor → edit a field → save: unchanged, completes with the contact identifier.
• Editor → close without changes: unchanged, completes with null.

Notes

showEditor still returns null for both "cancelled" and "deleted". Distinguishing them would need
an API change (e.g. a sentinel or a richer result), so this PR keeps the signature and just fixes
the stuck sheet — happy to follow up with that if you'd like it.