Add validation if SAML Keystore has no private key (#29847)

* initial changes

* add saml api

* address changes

* git restore enterprise/backend/test/metabase_enterprise/sso/api/saml_test.clj

* update comment

* update to have /api

* remove dbg

* fix alias

* change deps

* update test

* add test

* update test

* split tests

* add paren