The enable/disable toggle PUT sends back the flat list-response shape
(no nested oidc/saml objects). handleUpdateSSOProvider was unmarshaling
this directly, leaving OIDC and SAML as nil and overwriting all stored
credentials on every toggle.
Now preserves existing sub-config objects when the incoming payload omits
them, matching the existing ClientSecret preservation behaviour.
Fixes part of #1255
(cherry picked from commit 44868e99d66aa157f5c62d100151a6f8bc940205)
rcourtman
97aee77ae7