user_groups: Update edit_user_group codepath to send event on commit.

Earlier, we were using 'send_event' in 'edit_user_group' codepath
which can lead to a situation where we enqueue events but the
function fails at a later stage.

Events should not be sent until we know we're not rolling back.

Fixes part of #30489.

zerver/actions/user_groups.py

@@ -213,7 +213,7 @@ def do_send_user_group_update_event(
     user_group: NamedUserGroup, data: dict[str, str | int | AnonymousSettingGroupDict]
 ) -> None:
     event = dict(type="user_group", op="update", group_id=user_group.id, data=data)
-    send_event(user_group.realm, event, active_user_ids(user_group.realm_id))
+    send_event_on_commit(user_group.realm, event, active_user_ids(user_group.realm_id))
 
 
 @transaction.atomic(savepoint=False)

zerver/views/user_groups.py

@@ -95,7 +95,7 @@ def get_user_group(request: HttpRequest, user_profile: UserProfile) -> HttpRespo
     return json_success(request, data={"user_groups": user_groups})
 
 
-@transaction.atomic
+@transaction.atomic(durable=True)
 @require_user_group_edit_permission
 @typed_endpoint
 def edit_user_group(