Schema for the pgflow.step_tasks table.
Represents an individual task within a step (for map-type steps) with composite primary key (run_id, step_slug, task_index).
@type t() :: %PgFlow.Schema.StepTask{ __meta__: term(), error_message: term(), input: term(), message_id: term(), output: term(), run_id: term(), status: term(), step_slug: term(), step_state: term(), task_index: term() }