Save needlessly collecting list in InternalMappedSignificantTerms (el…

…astic#112502)

No need to allocate a list+stream to do an unsafe(ish) cast.

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/InternalMappedSignificantTerms.java

@@ -71,8 +71,9 @@ protected final void writeTermTypeInfoTo(StreamOutput out) throws IOException {
     }
 
     @Override
+    @SuppressWarnings({ "rawtypes", "unchecked" })
     public Iterator<SignificantTerms.Bucket> iterator() {
-        return buckets.stream().map(bucket -> (SignificantTerms.Bucket) bucket).toList().iterator();
+        return (Iterator) buckets.iterator();
     }
 
     @Override