Fixes#46641
When Fleet runs under a subpath, server_url already includes that
subpath, so appending url_prefix again produced a doubled ACS callback
path (e.g. https://host/subpath/subpath/api/v1/fleet/sso/callback),
breaking SAML authentication for both login and MDM end user
authentication.
Drop url_prefix from the callback URL construction so the path is
appended directly to server_url, which is the full external base URL.
Fixes the same flaw in all five ACS-construction sites: login SSO
initiate and callback, and MDM SSO initiate plus both callback branches.
Juan Fernandez
194f0cfb8f