Add an optional-group branch to runNodeAndTraverse beside foreach/loop: when
the group id is in the task's enabledWorkflowSteps it runs the template subgraph
exactly once (new single-pass runOptionalGroup helper, no rework/iteration);
otherwise it passes through to the group's children without executing the body.
Covered by a two-task divergence execution test plus single-pass, inertness,
and template-failure-routing cases.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
