Add RUN_ENGINE_DEQUEUE_DISABLED_WORKER_QUEUES: a comma-separated list of
worker queues (or base regions, which also cover the :scheduled split) for
which the engine API refuses worker dequeue requests and returns no work, so
those runs stay queued instead of being handed to workers that cannot run
them. Unset means no gating. Blocked dequeues increment the
run_engine.dequeue.blocked otel counter, tagged by worker_queue and region.