test: add ISO8601 round-trip tests for non-UTC timezone offsets

The _format_datetime_iso8601 offset formatting code (sign, hours,
minutes) was only tested with UTC (+00:00). Added tests for positive
(+05:30), negative (-07:00), and negative fractional (-09:30) offsets,
plus a string format verification test.

Closes #113

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

tests/test_types.py

@@ -379,6 +379,49 @@ def test_encode_decode_iso8601(self) -> None:
         assert decoded.minute == 30
         assert decoded.second == 45
 
+    def test_encode_decode_iso8601_positive_offset(self) -> None:
+        """113: non-UTC positive offset must round-trip correctly."""
+        import datetime
+
+        tz = datetime.timezone(datetime.timedelta(hours=5, minutes=30))
+        dt = datetime.datetime(2024, 1, 15, 10, 30, 0, tzinfo=tz)
+        encoded, vtype = encode_value(dt, ValueType.ISO8601)
+        assert vtype == ValueType.ISO8601
+        decoded, _ = decode_value(encoded, ValueType.ISO8601)
+        assert decoded == dt
+
+    def test_encode_decode_iso8601_negative_offset(self) -> None:
+        """113: non-UTC negative offset must round-trip correctly."""
+        import datetime
+
+        tz = datetime.timezone(datetime.timedelta(hours=-7))
+        dt = datetime.datetime(2024, 7, 4, 14, 0, 0, tzinfo=tz)
+        encoded, vtype = encode_value(dt, ValueType.ISO8601)
+        assert vtype == ValueType.ISO8601
+        decoded, _ = decode_value(encoded, ValueType.ISO8601)
+        assert decoded == dt
+
+    def test_encode_decode_iso8601_negative_fractional_offset(self) -> None:
+        """113: negative fractional-hour offset (-09:30) must round-trip."""
+        import datetime
+
+        tz = datetime.timezone(datetime.timedelta(hours=-9, minutes=-30))
+        dt = datetime.datetime(2024, 3, 1, 8, 0, 0, tzinfo=tz)
+        encoded, vtype = encode_value(dt, ValueType.ISO8601)
+        assert vtype == ValueType.ISO8601
+        decoded, _ = decode_value(encoded, ValueType.ISO8601)
+        assert decoded == dt
+
+    def test_format_iso8601_positive_offset_string(self) -> None:
+        """113: verify the formatted string for positive offset."""
+        import datetime
+
+        from dqlitewire.types import _format_datetime_iso8601
+
+        tz = datetime.timezone(datetime.timedelta(hours=5, minutes=30))
+        dt = datetime.datetime(2024, 1, 15, 10, 30, 0, tzinfo=tz)
+        assert _format_datetime_iso8601(dt) == "2024-01-15 10:30:00+05:30"
+
     def test_encode_decode_pre_epoch_datetime(self) -> None:
         """Pre-epoch datetimes (year < 1000) must roundtrip correctly.
         strftime('%Y') produces fewer than 4 digits on some platforms,