Get Step Run Rows

Head-sample of rows in this StepRun's step output.

Every step always runs full (incremental-rethink Task 10) — there is no per-run watermark window to slice by any more. This endpoint used to slice a (prior_watermark, this_watermark] window for incremental StepRuns; that branch was already dead in practice (nothing has written StepRun.output_watermark/execution_mode_used since Task 9) and the columns themselves are dropped in Task 13. Always returns a head-sample of the step's cumulative output parquet plus its total row count.