Initiate SAML Single Sign-On flow.
Redirects the user to the Identity Provider’s SSO URL with a SAML AuthnRequest.
Args: request: FastAPI request object. account: Account identifier (slug or ID). provider: Optional provider name to use (e.g., ‘okta’, ‘azure_ad’). If not specified, uses the configured provider for the account. relay_state: Optional state to return after successful authentication. session: Database session.
Returns: RedirectResponse to the IdP.