Reachability probe, present only when the resolved client supplies one
(M90b). The AWS SDK facade has no non-mutating probe of its own, so the
adapted (lazy) path stays undefined and the indicator reports
reachable: 'unknown'; an injected facade that exposes isHealthy
publishes real reachability.
Usage
import { AwsKmsProvider } from "secrets-plugin/src/index.ts";