Update the TimestampStrings zone info handling

time_zone is the string version of the time zone data
time_zone_zi is the ZoneInfo object of above

tests/unit/string_handling/test_timestamp_strings.py

@@ -144,8 +144,8 @@ class TestTimestampStrings:
             ts = TimestampStrings(time_zone=custom_tz_obj)
 
             # The time_zone should be the ZoneInfo object itself
-            assert ts.time_zone is custom_tz_obj
-            assert isinstance(ts.time_zone, ZoneInfo)
+            assert ts.time_zone_zi is custom_tz_obj
+            assert isinstance(ts.time_zone_zi, ZoneInfo)
 
     def test_zoneinfo_object_vs_string_equivalence(self):
         """Test that ZoneInfo object and string produce equivalent results"""
@@ -167,7 +167,7 @@ class TestTimestampStrings:
 
             # The time_zone attributes will be different types but represent the same timezone
             assert str(ts_string.time_zone) == 'Europe/Paris'
-            assert isinstance(ts_zoneinfo.time_zone, ZoneInfo)
+            assert isinstance(ts_zoneinfo.time_zone_zi, ZoneInfo)
 
     def test_edge_case_midnight(self):
         """Test timestamp formatting at midnight"""