TST: Add regression test for DatetimeIndex.union across DST boundary (#62915)

Author: zhan7236

pandas/tests/indexes/datetimes/test_setops.py

@@ -757,3 +757,13 @@ def test_intersection_non_nano_rangelike():
         freq="D",
     )
     tm.assert_index_equal(result, expected)
+
+
+def test_union_across_dst_boundary():
+    # GH#62915 union should work when one index ends at DST boundary
+    # and the other extends past it
+    index1 = date_range("2025-10-25", "2025-10-26", freq="D", tz="Europe/Helsinki")
+    index2 = date_range("2025-10-25", "2025-10-28", freq="D", tz="Europe/Helsinki")
+    result = index1.union(index2)
+    expected = date_range("2025-10-25", "2025-10-28", freq="D", tz="Europe/Helsinki")
+    tm.assert_index_equal(result, expected)