Authenticate Demo Session

Authenticate a demo session using session_id.

This endpoint validates the session_id and returns the demo user with auth token if the session is valid (created within last 30 minutes). The session is deleted after successful authentication. Can only be called from the demo tenant and only works for demo accounts.

Args: session_id: The session ID returned from create-demo-session

Returns: UserLoginOut: User information with auth token (same as /login)