Add a check to PUT /user/:id to disallow name edits if an SSO user (#23752)

* Add a check to PUT /user/:id to disallow name edits if an SSO user

* Clean up After SAML SSO tests

The `:sso_source` key is set for the Rasta user in some SAML tests, but is expeted to be nil in subsequent tests, so
we clean up in the SAML test ns.

* Add a test to ensure SSO user names can't be changed via API

* Missed a change I had made while adjusting tests

* valid-name-update? take 1 name, to allow better error messages

Let's the user know that first or last name is the cause of a problem, rather than just 'names'.

* Remove unneeded thread macro

* Use partial=

* slight change to local fn for reusability