Prevent 'stuck' gateways (without Namespace.Manage permission assigned)

[rustyjux]

API Services Portal Issue

Users can remove Namespace.Manage permission from themselves, even if they are the only user with this permission. This results in a gateway no one can manage the access on.

Fix: When updating user permissions to remove Namespace.Manage, confirm there is at least one other user with namespace.manage permission on the gateway. If not, prevent the submission and provide an error message.