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

@@ -421,10 +421,6 @@ public SortFieldAndFormat build(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);
                     nested = resolveNested(context, nestedSort);
                 } else {

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

@@ -626,10 +626,6 @@ public SortFieldAndFormat build(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");
-            }
             // new nested sorts takes priority
             validateMaxChildrenExistOnlyInTopLevelNestedSort(context, nestedSort);
             nested = resolveNested(context, nestedSort);

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

@@ -321,10 +321,6 @@ public SortFieldAndFormat build(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");
-            }
             // new nested sorts takes priority
             validateMaxChildrenExistOnlyInTopLevelNestedSort(context, nestedSort);
             nested = resolveNested(context, nestedSort);