Two fixes:

1. Move onStepDown() before WaitForMajorityService::shutDown() so in-flight waitForMajority calls get a clean cancellation instead of ShutdownInProgress, which withAutomaticRetry would retry and hit the kError→kError invariant in _transitionToError.
2. Add explicit interrupt() at the end of testProcessDonorFields/testProcessRecipientFields to prevent state machines from dispatching mock network requests before teardown, which deadlocked shutdownExecutorPool() under TSAN.