fix: [df52] route timestamp timezone mismatches through spark_parquet_convert (#3494)

INT96 Parquet timestamps are coerced to Timestamp(us, None) by DataFusion
but the logical schema expects Timestamp(us, Some("UTC")). The schema
adapter was routing this mismatch through Spark's Cast expression, which
incorrectly treats None-timezone values as TimestampNTZ (local time) and
applies a timezone conversion. This caused results to be shifted by the
session timezone offset (e.g., -5h45m for Asia/Kathmandu).

Route Timestamp->Timestamp mismatches through CometCastColumnExpr which
delegates to spark_parquet_convert, handling this as a metadata-only
timezone relabel without modifying the underlying values.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: andygrove

native/core/src/parquet/schema_adapter.rs

@@ -194,14 +194,23 @@ impl SparkPhysicalExprAdapter {
             let physical_type = cast.input_field().data_type();
             let target_type = cast.target_field().data_type();
 
-            // For complex nested types (Struct, List, Map), use CometCastColumnExpr
-            // with spark_parquet_convert which handles field-name-based selection,
-            // reordering, and nested type casting correctly.
+            // For complex nested types (Struct, List, Map) and Timestamp timezone
+            // mismatches, use CometCastColumnExpr with spark_parquet_convert which
+            // handles field-name-based selection, reordering, nested type casting,
+            // and metadata-only timestamp timezone relabeling correctly.
+            //
+            // Timestamp mismatches (e.g., Timestamp(us, None) -> Timestamp(us, Some("UTC")))
+            // occur when INT96 Parquet timestamps are coerced to Timestamp(us, None) by
+            // DataFusion but the logical schema expects Timestamp(us, Some("UTC")).
+            // Using Spark's Cast here would incorrectly treat the None-timezone values as
+            // local time (TimestampNTZ) and apply a timezone conversion, but the values are
+            // already in UTC. spark_parquet_convert handles this as a metadata-only change.
             if matches!(
                 (physical_type, target_type),
                 (DataType::Struct(_), DataType::Struct(_))
                     | (DataType::List(_), DataType::List(_))
                     | (DataType::Map(_, _), DataType::Map(_, _))
+                    | (DataType::Timestamp(_, _), DataType::Timestamp(_, _))
             ) {
                 let comet_cast: Arc<dyn PhysicalExpr> = Arc::new(
                     CometCastColumnExpr::new(