Fix backport of #46731 (#52744)

This change fixes the incomplete backport of #46731 in 7.x (as of 7.5).
We now check if `max_children` is set on the top level nested sort and fails with an
exception if it's not the case.

Relates #46731
Closes #52202

Author: jimczi

server/src/main/java/org/elasticsearch/search/sort/FieldSortBuilder.java

@@ -476,10 +476,6 @@ private Nested nested(QueryShardContext context, MappedFieldType fieldType) thro
             throw new QueryShardException(context,
                 "max_children is only supported on v6.5.0 or higher");
         }
-        if (nestedSort.getNestedSort() != null && nestedSort.getMaxChildren() != Integer.MAX_VALUE)  {
-            throw new QueryShardException(context,
-                "max_children is only supported on last level of nested sort");
-        }
         validateMaxChildrenExistOnlyInTopLevelNestedSort(context, nestedSort);
         return resolveNested(context, nestedSort);
     }

server/src/main/java/org/elasticsearch/search/sort/GeoDistanceSortBuilder.java

@@ -605,7 +605,7 @@ public BucketedSort buildBucketedSort(QueryShardContext context) throws IOExcept
         IndexGeoPointFieldData geoIndexFieldData = fieldData(context);
         Nested nested = nested(context);
 
-        // TODO implement the single point optimization above 
+        // TODO implement the single point optimization above
 
         return comparatorSource(localPoints, localSortMode, geoIndexFieldData, nested)
                 .newBucketedSort(context.bigArrays(), order, DocValueFormat.RAW);
@@ -670,10 +670,6 @@ private Nested nested(QueryShardContext context) throws IOException {
             throw new QueryShardException(context,
                 "max_children is only supported on v6.5.0 or higher");
         }
-        if (nestedSort.getNestedSort() != null && nestedSort.getMaxChildren() != Integer.MAX_VALUE)  {
-            throw new QueryShardException(context,
-                "max_children is only supported on last level of nested sort");
-        }
         validateMaxChildrenExistOnlyInTopLevelNestedSort(context, nestedSort);
         return resolveNested(context, nestedSort);
     }

server/src/main/java/org/elasticsearch/search/sort/ScriptSortBuilder.java

@@ -332,10 +332,6 @@ private IndexFieldData.XFieldComparatorSource fieldComparatorSource(QueryShardCo
                 throw new QueryShardException(context,
                     "max_children is only supported on v6.5.0 or higher");
             }
-            if (nestedSort.getNestedSort() != null && nestedSort.getMaxChildren() != Integer.MAX_VALUE)  {
-                throw new QueryShardException(context,
-                    "max_children is only supported on last level of nested sort");
-            }
             // new nested sorts takes priority
             validateMaxChildrenExistOnlyInTopLevelNestedSort(context, nestedSort);
             nested = resolveNested(context, nestedSort);