The runtime's ServerEffectHandlers.callService no-op'd when no consumer
handler was supplied, breaking offline-preview / runtime-verify chains
that route through serverHandlers (when `persistence` is set, the UI
hook prefers serverHandlers' callService over clientHandlers'). Mirror
the same mock-result fallback so service-atom chains advance instead
of emitting PAYMENT_CREATED with null payload.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>