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

[mcruzdev]

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.

Closes #1625

[Copilot]

Pull request overview

This PR improves validation/diagnostics for programmatically-built workflows by ensuring a missing required for.in value in a ForTask fails fast with a descriptive message (instead of a contextless NullPointerException), and adds a regression test for that behavior.

Changes:

• Add Objects.requireNonNull(...) around task.getFor().getIn() in ForExecutorBuilder.buildCollectionFilter() with a clear error message.
• Add a JUnit test that builds a ForTask without in and asserts the thrown exception includes the descriptive message.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
impl/core/src/main/java/io/serverlessworkflow/impl/executors/ForExecutor.java	Adds a null check for required for.in and throws an NPE with a descriptive message.
impl/test/src/test/java/io/serverlessworkflow/impl/test/ForTaskMissingInTest.java	Adds regression coverage ensuring missing in fails with a helpful message.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

impl/core/src/main/java/io/serverlessworkflow/impl/executors/ForExecutor.java:56

• buildCollectionFilter() still dereferences task.getFor() without a null check. The schema requires for (in addition to for.in), and programmatic workflows can bypass schema validation, so a missing for will still throw a contextless NPE. Consider validating task.getFor() explicitly before accessing .getIn() so missing for fails with a descriptive message too.

    protected WorkflowValueResolver<Collection<?>> buildCollectionFilter() {
      In in =
          Objects.requireNonNull(task.getFor().getIn(), "'in' is a required property for ForTask");

[mcruzdev]

cc: @ricardozanini, it is important for 1.0.0