fix EsqlNodeSubclassTests

Author: ChrisHegarty

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/logical/Fork.java

@@ -29,6 +29,9 @@ public class Fork extends UnaryPlan implements SurrogateLogicalPlan {
 
     public Fork(Source source, LogicalPlan child, List<LogicalPlan> subPlans) {
         super(source, child);
+        if (subPlans.size() < 2) {
+            throw new IllegalArgumentException("requires more than two subqueries, got:" + subPlans.size());
+        }
         this.subPlans = subPlans;
     }
 

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/tree/EsqlNodeSubclassTests.java

@@ -146,7 +146,7 @@ public static List<Object[]> nodeSubclasses() throws IOException {
             .toList();
     }
 
-    private static final List<Class<?>> CLASSES_WITH_MIN_TWO_CHILDREN = List.of(Concat.class, CIDRMatch.class);
+    private static final List<Class<?>> CLASSES_WITH_MIN_TWO_CHILDREN = List.of(Concat.class, CIDRMatch.class, Fork.class, Merge.class);
 
     // List of classes that are "unresolved" NamedExpression subclasses, therefore not suitable for use with logical/physical plan nodes.
     private static final List<Class<?>> UNRESOLVED_CLASSES = List.of(