Make DfsPhase a utility class (elastic#112553)

This thing has no state, no need to have an instance for it.

Author: original-brownbear

server/src/main/java/org/elasticsearch/search/SearchService.java

@@ -278,8 +278,6 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
 
     private final BigArrays bigArrays;
 
-    private final DfsPhase dfsPhase = new DfsPhase();
-
     private final FetchPhase fetchPhase;
     private final RankFeatureShardPhase rankFeatureShardPhase;
     private volatile boolean enableSearchWorkerThreads;
@@ -511,7 +509,7 @@ private DfsSearchResult executeDfsPhase(ShardSearchRequest request, SearchShardT
             Releasable ignored = readerContext.markAsUsed(getKeepAlive(request));
             SearchContext context = createContext(readerContext, request, task, ResultsType.DFS, false)
         ) {
-            dfsPhase.execute(context);
+            DfsPhase.execute(context);
             return context.dfsResult();
         } catch (Exception e) {
             logger.trace("Dfs phase failed", e);

server/src/main/java/org/elasticsearch/search/dfs/DfsPhase.java

@@ -52,7 +52,9 @@
  */
 public class DfsPhase {
 
-    public void execute(SearchContext context) {
+    private DfsPhase() {}
+
+    public static void execute(SearchContext context) {
         try {
             collectStatistics(context);
             executeKnnVectorQuery(context);
@@ -65,7 +67,7 @@ public void execute(SearchContext context) {
         }
     }
 
-    private void collectStatistics(SearchContext context) throws IOException {
+    private static void collectStatistics(SearchContext context) throws IOException {
         final DfsProfiler profiler = context.getProfilers() == null ? null : context.getProfilers().getDfsProfiler();
 
         Map<String, CollectionStatistics> fieldStatistics = new HashMap<>();