Force execution of fetch tasks (#31974)

Forces fetch tasks to queue even in the event that the queue is
already full. The reasoning is that fetch tasks may only be follow-up
to query tasks, so the number of additional fetch tasks that may enter
the threadpool is expected to be reasonable.

Closes #29442

Author: imotov

server/src/main/java/org/elasticsearch/action/search/SearchTransportService.java

@@ -409,7 +409,7 @@ public void onFailure(Exception e) {
             });
         TransportActionProxy.registerProxyAction(transportService, FETCH_ID_SCROLL_ACTION_NAME, FetchSearchResult::new);
 
-        transportService.registerRequestHandler(FETCH_ID_ACTION_NAME, ThreadPool.Names.SEARCH, ShardFetchSearchRequest::new,
+        transportService.registerRequestHandler(FETCH_ID_ACTION_NAME, ThreadPool.Names.SEARCH, true, true, ShardFetchSearchRequest::new,
             (request, channel, task) -> {
                 FetchSearchResult result = searchService.executeFetchPhase(request, (SearchTask)task);
                 channel.sendResponse(result);