[Fix #1625] Add null check for required 'in' property in ForExecutorBuilder

ForExecutorBuilder.buildCollectionFilter() dereferences task.getFor().getIn()
without a null check. Programmatically-built workflows bypass schema validation,
so a ForTask without 'in' set causes a raw NullPointerException with no context.

Added Objects.requireNonNull with a descriptive message and a test that verifies
the error when 'in' is missing.

Signed-off-by: Matheus Cruz <matheuscruz.dev@gmail.com>

Author: mcruzdev

impl/core/src/main/java/io/serverlessworkflow/impl/executors/ForExecutor.java

@@ -28,6 +28,7 @@
 import io.serverlessworkflow.impl.expressions.ExpressionDescriptor;
 import java.util.Collection;
 import java.util.Iterator;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.concurrent.CompletableFuture;
 
@@ -51,7 +52,8 @@ protected Optional<WorkflowPredicate> buildWhileFilter() {
     }
 
     protected WorkflowValueResolver<Collection<?>> buildCollectionFilter() {
-      In in = task.getFor().getIn();
+      In in =
+          Objects.requireNonNull(task.getFor().getIn(), "'in' is a required property for ForTask");
       return application
           .expressionFactory()
           .resolveCollection(

impl/test/src/test/java/io/serverlessworkflow/impl/test/ForTaskMissingInTest.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2020-Present The Serverless Workflow Specification Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.serverlessworkflow.impl.test;
+
+import static io.serverlessworkflow.fluent.spec.dsl.DSL.*;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import io.serverlessworkflow.api.types.Workflow;
+import io.serverlessworkflow.fluent.spec.WorkflowBuilder;
+import io.serverlessworkflow.impl.WorkflowApplication;
+import org.junit.jupiter.api.Test;
+
+class ForTaskMissingInTest {
+
+  @Test
+  void forTaskWithoutInShouldFailWithDescriptiveMessage() {
+    Workflow workflow =
+        WorkflowBuilder.workflow("for-missing-in", "test", "0.1.0")
+            .tasks(
+                doTasks(
+                    forEach(
+                        "loopWithoutIn",
+                        f -> f.each("item").tasks(t -> t.set("noop", s -> s.put("done", true))))))
+            .build();
+    try (WorkflowApplication app = WorkflowApplication.builder().build()) {
+      assertThatThrownBy(() -> app.workflowDefinition(workflow))
+          .isInstanceOf(NullPointerException.class)
+          .hasMessageContaining("'in' is a required property for ForTask");
+    }
+  }
+}