MaskingResolverException
Thrown by DefaultMaskingEngine when the policy resolver fails and the engine is configured with ResolverErrorMode.FAIL_REQUEST, so a resolver outage surfaces as an error (an adapter maps it to a 5xx) instead of a silently masked {} that reports success.
The MaskingFailureSink is notified with FailReason.RESOLVER_ERROR before this is thrown, so fail-request is strictly more observable than mask-all, never less. See ADR 0011.