Add matrix stats test for MultiValueMode differences

Added SearchIT.testMatrixStatsMultiValueModeEffect to verify that different
MultiValueMode settings (AVG vs MIN) produce different matrix stats results
for multi-valued fields.

Signed-off-by: Jinwoo Lee <jinlee1703@gmail.com>

Author: jinlee1703

client/rest-high-level/src/test/java/org/opensearch/client/SearchIT.java

@@ -78,6 +78,7 @@
 import org.opensearch.script.mustache.MultiSearchTemplateResponse.Item;
 import org.opensearch.script.mustache.SearchTemplateRequest;
 import org.opensearch.script.mustache.SearchTemplateResponse;
+import org.opensearch.search.MultiValueMode;
 import org.opensearch.search.SearchHit;
 import org.opensearch.search.aggregations.AggregationBuilders;
 import org.opensearch.search.aggregations.BucketOrder;
@@ -1827,6 +1828,40 @@ public void testCountAllIndicesMatchQuery() throws IOException {
         assertEquals(3, countResponse.getCount());
     }
 
+    public void testMatrixStatsMultiValueModeEffect() throws IOException {
+        Request createIndex = new Request(HttpPut.METHOD_NAME, "/test_multi");
+        client().performRequest(createIndex);
+
+        Request doc = new Request(HttpPut.METHOD_NAME, "/test_multi/_doc/1");
+        doc.setJsonEntity("{ \"num\": [10, 30], \"num2\": [40, 60] }");
+        client().performRequest(doc);
+
+        client().performRequest(new Request(HttpPost.METHOD_NAME, "/test_multi/_refresh"));
+
+        SearchRequest avgRequest = new SearchRequest("test_multi");
+        avgRequest.source(
+            new SearchSourceBuilder().aggregation(
+                new MatrixStatsAggregationBuilder("agg").fields(Arrays.asList("num", "num2")).multiValueMode(MultiValueMode.AVG)
+            ).size(0)
+        );
+
+        SearchRequest minRequest = new SearchRequest("test_multi");
+        minRequest.source(
+            new SearchSourceBuilder().aggregation(
+                new MatrixStatsAggregationBuilder("agg").fields(Arrays.asList("num", "num2")).multiValueMode(MultiValueMode.MIN)
+            ).size(0)
+        );
+
+        SearchResponse avgResponse = execute(avgRequest, highLevelClient()::search, highLevelClient()::searchAsync);
+        SearchResponse minResponse = execute(minRequest, highLevelClient()::search, highLevelClient()::searchAsync);
+
+        MatrixStats avgStats = avgResponse.getAggregations().get("agg");
+        MatrixStats minStats = minResponse.getAggregations().get("agg");
+
+        assertNotEquals(avgStats.getMean("num"), minStats.getMean("num"));
+        assertNotEquals(avgStats.getMean("num2"), minStats.getMean("num2"));
+    }
+
     private static void assertCountHeader(CountResponse countResponse) {
         assertEquals(0, countResponse.getSkippedShards());
         assertEquals(0, countResponse.getFailedShards());