add unit test for projecting timestamp to timestamptz

Author: kevinjqliu

tests/io/test_pyarrow.py

@@ -2521,6 +2521,45 @@ def test_initial_value() -> None:
         assert val.as_py() == 22
 
 
+def test__to_requested_schema_timestamp_to_timestamptz_projection() -> None:
+    from datetime import datetime, timezone
+
+    # file is written with timestamp without timezone
+    file_schema = Schema(NestedField(1, "ts_field", TimestampType(), required=False))
+    batch = pa.record_batch(
+        [
+            pa.array(
+                [
+                    datetime(2025, 8, 14, 12, 0, 0),
+                    datetime(2025, 8, 14, 13, 0, 0),
+                ],
+                type=pa.timestamp("us"),
+            )
+        ],
+        names=["ts_field"],
+    )
+
+    # table is written with timestamp with timezone
+    table_schema = Schema(NestedField(1, "ts_field", TimestamptzType(), required=False))
+
+    actual_result = _to_requested_schema(table_schema, file_schema, batch, downcast_ns_timestamp_to_us=True)
+    expected = pa.record_batch(
+        [
+            pa.array(
+                [
+                    datetime(2025, 8, 14, 12, 0, 0),
+                    datetime(2025, 8, 14, 13, 0, 0),
+                ],
+                type=pa.timestamp("us", tz=timezone.utc),
+            )
+        ],
+        names=["ts_field"],
+    )
+
+    # expect actual_result to have timezone
+    assert expected.equals(actual_result)
+
+
 def test__to_requested_schema_timestamps(
     arrow_table_schema_with_all_timestamp_precisions: pa.Schema,
     arrow_table_with_all_timestamp_precisions: pa.Table,