Creates or updates the split test configuration on an unsent broadcast, defining how much of the audience samples each variant before a winner is chosen. mode accepts only offer_test. sample_percent and evaluate_after_minutes must be supplied together or omitted together. preferred_metric overrides the default winner-metric cascade; omit it to leave the cascade in place. Split tests run one level deep -- configuring a broadcast that is itself a variant is rejected. Once any variant exists, reconfiguring is rejected if the audience is too small to support one more arm. The configuration locks once the split test has launched.