#966 Branch on_failure handlers never run; handler referenced inside a parallel branch lambda is auto-chained in the parent and runs unconditionally

EARS: When a task inside a parallel branch fails and declares on_failure, the engine shall run that handler for that failure only. Today the handler is not in branch_workflows, so _handle_failure_exception finds no callback and the branch fails; WorkflowBuilder does not recognise the handler (referenced inside the lambda) and auto-chains it after the previous parent task, so it runs whether or not the branch failed (golden v2 compensation_task, golden disaster phase3_compensation_task, DSL_PROMPT parallel_with_compensation example). Found during #955.