expiresInSeconds method
Returns the number of whole seconds until expiresAt, mirroring the
OAuth 2.0 expires_in field used in token responses.
Returns null when expiresAt is not set. Returns 0 (never negative)
when the token is already expired so callers can re-serialize a token
without producing an invalid expires_in.
Implementation
int? expiresInSeconds([DateTime? now]) => remainingLifetime(now)?.inSeconds;