make it clear that seed is for testing

Author: jan-elastic

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/SampleOperator.java

@@ -28,12 +28,21 @@
 
 public class SampleOperator implements Operator {
 
-    public record Factory(double probability, Integer seed) implements OperatorFactory {
+    public static class Factory implements OperatorFactory {
+
+        private final double probability;
+        private final Integer seed;
 
         public Factory(double probability) {
             this(probability, null);
         }
 
+        // visible for testing
+        Factory(double probability, Integer seed) {
+            this.probability = probability;
+            this.seed = seed;
+        }
+
         @Override
         public SampleOperator get(DriverContext driverContext) {
             return new SampleOperator(probability, seed == null ? Randomness.get().nextInt() : seed);