Log examples arrow failure (#3645)

Follow up: #3482 - when arrow conversion fails - print a couple of
records.

Author: ravwojdyla

lib/zephyr/src/zephyr/execution.py

@@ -396,10 +396,14 @@ def _write_stage_chunks(
             use_parquet = True
             logger.info("Using Parquet for scatter serialization for shard %d", source_shard)
         except Exception:
+            sample_rows = str(test_envelope[:5]) if len(test_envelope) > 5 else str(test_envelope)
+            if len(sample_rows) > 1000:
+                sample_rows = sample_rows[:1000] + "...(truncated)"
             logger.warning(
                 "Arrow scatter serialization failed for shard %d; "
-                "falling back to pickle. Performance will be degraded.",
+                "falling back to pickle. Performance will be degraded. Sample rows: %s",
                 source_shard,
+                sample_rows,
                 exc_info=True,
             )