Retrieves a single row from an audience by its rowId. Returns the full row record including all cell values and their CellState — RESULT (value present), BLANK (no value set), LOADING (enrichment in progress), ERROR (enrichment failed for this cell), or NO_RESULT (enrichment completed but returned no match). To limit the response to specific columns, pass one or more columnId values in the columns query parameter; if omitted, all columns are returned. Use this endpoint to inspect individual records, verify enrichment results, or retrieve a row before an update operation. Returns 200 with the row resource. Returns 404 if the audienceId or rowId does not exist.