* fix(sso): share SAML sign-in state over Redis without enable_redis_auth_cache
SAML login tickets, the single-use assertion mark and cached IdP metadata
lived in user_api_key_cache, which only gets Redis when
enable_redis_auth_cache is on. With several replicas or workers the IdP
callback often landed on a process that never saw the ticket and got a
401, and IdP-initiated sign-in was refused even with Redis configured.
SAML state now has its own DualCache that _attach_redis_usage_cache
always wires to Redis, like the CLI SSO session cache, and the SAML cache
calls are async.
* test(sso): drop docstring from the SAML proxy pair fixture
* test(sso): cover the SAML login and metadata endpoints
