Starts a SAML2 SSO login by forwarding the request to Backend API v2.
After a user logs in successfully, a session is created and a JWT token is returned.
This token represents the user's authenticated session.
It must be included in the Authorization header as a Bearer token for all protected endpoints:
Authorization: Bearer <your-token>
The server will validate this token to authorize access.
Contains the IdP redirect URL and relay state.