feat(dataframe): add synchronous wrapper for SendableRecordBatchStream

Author: kosiew

src/dataframe.rs

@@ -353,7 +353,13 @@ impl PyDataFrame {
         Ok(html_str)
     }
 }
-
+/// Synchronous wrapper around a [`SendableRecordBatchStream`] used for
+/// the `__arrow_c_stream__` implementation.
+///
+/// It uses `runtime.block_on` to consume the underlying async stream,
+/// providing synchronous iteration. When a `projection` is set, each
+/// batch is converted via `record_batch_into_schema` to apply schema
+/// changes per batch.
 struct DataFrameStreamReader {
     stream: SendableRecordBatchStream,
     runtime: Handle,