Three interrelated issues here:

- [ ] `TransportBroadcastByNodeAction` derivatives are typically executed via `NodeClient` which bypasses the `TransportService` and therefore does not respect (or even know about) the `executor` parameter which would fork. This means that the coordination work, including grouping all the shards by node, happens on the calling thread. When executed from the REST layer, that's a transport worker. This is #97916, but until that's fixed we need a workaround.

- [ ] The node-level responses are deserialised and processed on the receiving transport worker too.

- [ ] Several of these actions use `SAME` for their executor, bypassing the forking that _does_ exist today.

Relates #77466 
server/src/main/java/org/elasticsearch/action/support/broadcast/node/TransportBroadcastByNodeAction.java
List<ShardRouting> getShards() {
            return shards;
        }
        String getNodeId() {
            return nodeId;
        }
        Request getIndicesLevelRequest() {
            return indicesLevelRequest;
        }
        @Override
        public String[] indices() {
            return indicesLevelRequest.indices();
        }
        @Override
        public IndicesOptions indicesOptions() {
            return indicesLevelRequest.indicesOptions();
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            super.writeTo(out);
            indicesLevelRequest.writeTo(out);
            out.writeList(shards);
            out.writeString(nodeId);
        }
        @Override
        public Task createTask(long id, String type, String action, TaskId parentTaskId, Map<String, String> headers) {
            return indicesLevelRequest.createTask(id, type, action, parentTaskId, headers);
        }
    }
    class NodeResponse extends TransportResponse {