ForExecutorBuilder missing null check on for.in (required field) #1625

ForExecutorBuilder.buildCollectionFilter() dereferences task.getFor().getIn() without a null check. When in is null (a ForTask built programmatically without setting in), this throws a raw NullPointerException with no helpful context.

Per the Open Workflow specification, for.in is a required field. The JSON Schema in
types/src/main/resources/schema/workflow.yaml correctly declares required: [ in ], and the ValidationReader enforces this for YAML/JSON workflows.

However, programmatically-built workflows bypass schema validation entirely, so the in requirement is not enforced.