reuse bucket when reduce terms buckets

Signed-off-by: bowenlan-amzn <bowenlan23@gmail.com>

Author: bowenlan-amzn

server/src/main/java/org/opensearch/search/aggregations/bucket/terms/InternalTerms.java

@@ -550,13 +550,20 @@ protected B reduceBucket(List<B> buckets, ReduceContext context) {
             }
         }
 
-        InternalAggregations subAggs;
+        InternalAggregations reducedSubAggs;
         if (aggregationsList.isEmpty()) {
-            subAggs = InternalAggregations.EMPTY;
+            reducedSubAggs = InternalAggregations.EMPTY;
         } else {
-            subAggs = InternalAggregations.reduce(aggregationsList, context);
+            reducedSubAggs = InternalAggregations.reduce(aggregationsList, context);
         }
-        return createBucket(docCount, subAggs, docCountError, buckets.get(0));
+
+        // In-place mutation: reuse first bucket instead of allocating new one
+        B firstBucket = buckets.get(0);
+        Bucket<?> mutableBucket = (Bucket<?>) firstBucket;
+        mutableBucket.docCount = docCount;
+        mutableBucket.aggregations = reducedSubAggs;
+        mutableBucket.docCountError = docCountError;
+        return firstBucket;
     }
 
     protected abstract void setDocCountError(long docCountError);