Function explain_plan_inner

pub(crate) async fn explain_plan_inner(
    session: &Session,
    catalog: &Catalog,
    df_meta: DataflowMetainfo,
    explain_ctx: ExplainPlanContext,
    optimizer: Optimizer,
    insights_ctx: Option<Box<PlanInsightsContext>>,
) -> Result<Vec<Row>, AdapterError>

Generates EXPLAIN PLAN output. (Shared helper fn between the old and new sequencing.)