[multi-stage] [bug] Executor service is never shut down for QueryRunner

MailboxService is shutdown before executor service. Executor is never shut down.
This will crash pinot server during shut down and not sending upstream a shut down signal for upstream grpc channel.

Start with the QueryRunner shutdown path and inspect how MailboxService and the executor service are stopped. Reproduce or trace Pinot server shutdown to confirm the executor is terminated and the upstream gRPC channel receives its shutdown signal; add or update coverage for this sequence if the existing tests expose it.

Written by the indexing model from the issue text.