test: add coverage for naive datetime ISO8601 assume-UTC fallback

The _format_datetime_iso8601 function silently assumes UTC for naive
(timezone-unaware) datetimes by appending +00:00. This path had no
test coverage. Add tests documenting the behavior: UTC assumption,
microsecond preservation, and the naive-in/aware-out round-trip
asymmetry.

Closes #166.

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

Author: antoineleclair

tests/test_types.py

@@ -843,3 +843,40 @@ def test_encode_value_false_as_explicit_integer(self) -> None:
         encoded, vtype = encode_value(False, ValueType.INTEGER)
         assert vtype == ValueType.INTEGER
         assert decode_int64(encoded) == 0
+
+
+class TestNaiveDatetimeISO8601:
+    """166: naive datetime 'assume UTC' fallback path is untested."""
+
+    def test_naive_datetime_assumes_utc(self) -> None:
+        """Naive datetime is encoded as UTC (+00:00)."""
+        import datetime
+
+        naive = datetime.datetime(2024, 6, 15, 12, 30, 45)  # noqa: DTZ001
+        encoded, vtype = encode_value(naive, ValueType.ISO8601)
+        assert vtype == ValueType.ISO8601
+        decoded, _ = decode_value(encoded, ValueType.ISO8601)
+        assert decoded.utcoffset() == datetime.timedelta(0)
+        assert decoded.year == 2024
+        assert decoded.month == 6
+        assert decoded.hour == 12
+
+    def test_naive_datetime_with_microseconds(self) -> None:
+        """Naive datetime with fractional seconds encodes correctly."""
+        import datetime
+
+        naive = datetime.datetime(2024, 1, 1, 0, 0, 0, 123456)  # noqa: DTZ001
+        encoded, _ = encode_value(naive, ValueType.ISO8601)
+        decoded, _ = decode_value(encoded, ValueType.ISO8601)
+        assert decoded.microsecond == 123456
+
+    def test_naive_datetime_roundtrip_becomes_aware(self) -> None:
+        """Naive datetime round-trips as timezone-aware UTC."""
+        import datetime
+
+        naive = datetime.datetime(2024, 6, 15, 12, 30, 45)  # noqa: DTZ001
+        assert naive.tzinfo is None
+        encoded, _ = encode_value(naive, ValueType.ISO8601)
+        decoded, _ = decode_value(encoded, ValueType.ISO8601)
+        assert decoded.tzinfo is not None
+        assert decoded.replace(tzinfo=None) == naive