Require a fresh heartbeat after changing delivery fixtures

When an integration test shares a notification receiver across cases, changing its destination can silently invalidate a later watchdog test. A remembered heartbeat may then satisfy its start condition even though the watchdog no longer reaches the intended endpoint. Restore shared contact configuration after each mutation and require a new receipt observed after the next test begins, before exercising the stopped-sender path. This protects the causal test boundary; it does not establish production delivery or prove that every shared fixture is isolated.

Applies when: Delivery acceptance reuses contacts and receiver state across ordered tests. Keywords: watchdog heartbeat stale receipt shared fixture destination restore isolation