Fix matrix_stats aggregation cache conflict by including multiValueMode in equals/hashCode

- Added multiValueMode to equals() and hashCode() of MatrixStatsAggregationBuilder
- Added serialization/deserialization logic for multiValueMode in writeTo/readFrom
- Prevents incorrect aggregator reuse when aggregation mode changes (e.g. AVG ↔ MIN)

Author: jinlee1703

modules/aggs-matrix-stats/src/main/java/org/opensearch/search/aggregations/matrix/stats/MatrixStatsAggregationBuilder.java

@@ -45,6 +45,7 @@
 
 import java.io.IOException;
 import java.util.Map;
+import java.util.Objects;
 
 public class MatrixStatsAggregationBuilder extends ArrayValuesSourceAggregationBuilder.LeafOnly<MatrixStatsAggregationBuilder> {
     public static final String NAME = "matrix_stats";
@@ -74,22 +75,19 @@ protected AggregationBuilder shallowCopy(AggregatorFactories.Builder factoriesBu
      */
     public MatrixStatsAggregationBuilder(StreamInput in) throws IOException {
         super(in);
+        this.multiValueMode = in.readEnum(MultiValueMode.class);
     }
 
     @Override
-    protected void innerWriteTo(StreamOutput out) {
-        // Do nothing, no extra state to write to stream
+    protected void innerWriteTo(StreamOutput out) throws IOException {
+        out.writeEnum(multiValueMode);
     }
 
     public MatrixStatsAggregationBuilder multiValueMode(MultiValueMode multiValueMode) {
         this.multiValueMode = multiValueMode;
         return this;
     }
 
-    public MultiValueMode multiValueMode() {
-        return this.multiValueMode;
-    }
-
     @Override
     protected MatrixStatsAggregatorFactory innerBuild(
         QueryShardContext queryShardContext,
@@ -110,4 +108,18 @@ public XContentBuilder doXContentBody(XContentBuilder builder, ToXContent.Params
     public String getType() {
         return NAME;
     }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) return true;
+        if (obj == null || getClass() != obj.getClass()) return false;
+        if (super.equals(obj) == false) return false;
+        MatrixStatsAggregationBuilder other = (MatrixStatsAggregationBuilder) obj;
+        return multiValueMode == other.multiValueMode;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(super.hashCode(), multiValueMode);
+    }
 }