test(subgraph): cover RC handlers and handleOfferStored idempotency

Previously no tests exercised the RecurringCollector data source handlers
or the immutability guard on the Offer entity. Adds matchstick coverage for
handleAgreementAccepted, handleAgreementCanceled (both canceledBy variants +
early-return), handleAgreementUpdated (early-return + field updates),
handleRCACollected (early-return + accumulation), and handleOfferStored
(first-wins + duplicate event does not halt on immutable re-write).

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

Author: MoonBoi9001