Fix double array for sub_searches in toXContent (#96963)

This fixes two bugs:

* toXContent was writing two arrays instead of one array for sub_searches as part of 
SearchSourceBuilder
* Removes sub_searches from ShardSearchRequestTests.setForceSyntheticSource

Closes #96896
Closes #96910

Author: jdconrad

server/src/main/java/org/elasticsearch/search/builder/SearchSourceBuilder.java

@@ -1616,7 +1616,7 @@ public XContentBuilder innerToXContent(XContentBuilder builder, Params params) t
             if (subSearchSourceBuilders.size() == 1) {
                 builder.field(QUERY_FIELD.getPreferredName(), subSearchSourceBuilders.get(0).getQueryBuilder());
             } else {
-                builder.array(SUB_SEARCHES_FIELD.getPreferredName(), subSearchSourceBuilders);
+                builder.xContentList(SUB_SEARCHES_FIELD.getPreferredName(), subSearchSourceBuilders);
             }
         }
 

server/src/test/java/org/elasticsearch/action/get/MultiGetShardRequestTests.java

@@ -99,7 +99,6 @@ protected MultiGetShardRequest mutateInstance(MultiGetShardRequest instance) thr
         }
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/96910")
     public void testForceSyntheticUnsupported() {
         MultiGetShardRequest request = createTestInstance(true);
         StreamOutput out = new BytesStreamOutput();

server/src/test/java/org/elasticsearch/search/internal/ShardSearchRequestTests.java

@@ -40,6 +40,7 @@
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -257,11 +258,13 @@ public void testWillCallRequestCacheKeyDifferentiators() throws IOException {
         assertThat(invoked.get(), is(true));
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/96910")
     public void testForceSyntheticUnsupported() throws IOException {
         SearchRequest request = createSearchRequest();
         if (request.source() != null) {
             request.source().rankBuilder(null);
+            if (request.source().subSearches().size() >= 2) {
+                request.source().subSearches(new ArrayList<>());
+            }
         }
         request.setForceSyntheticSource(true);
         ShardSearchRequest shardRequest = createShardSearchReqest(request);