fix: zero-pad year in ISO 8601 datetime formatting

`_format_datetime_iso8601` used `strftime("%Y")` which produces
fewer than 4 digits for years before 1000 on Linux/glibc. The
resulting string (e.g., "1-01-01 00:00:00+00:00") cannot be parsed
back by `fromisoformat()`, breaking roundtrip for pre-epoch dates.

Use `f"{value.year:04d}"` for the year field to ensure 4-digit
years per ISO 8601.

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

Author: antoineleclair

src/dqlitewire/types.py

@@ -149,7 +149,7 @@ def decode_blob(data: bytes) -> tuple[bytes, int]:
 
 def _format_datetime_iso8601(value: datetime.datetime) -> str:
     """Format a datetime to ISO 8601 string matching Go's time format."""
-    formatted = value.strftime("%Y-%m-%d %H:%M:%S")
+    formatted = f"{value.year:04d}" + value.strftime("-%m-%d %H:%M:%S")
     if value.microsecond:
         formatted += f".{value.microsecond:06d}".rstrip("0")
     utcoffset = value.utcoffset()

tests/test_types.py

@@ -379,6 +379,22 @@ def test_encode_decode_iso8601(self) -> None:
         assert decoded.minute == 30
         assert decoded.second == 45
 
+    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,
+        which breaks fromisoformat on decode.
+        """
+        import datetime
+
+        dt = datetime.datetime(1, 1, 1, tzinfo=datetime.UTC)
+        encoded, vtype = encode_value(dt)
+        assert vtype == ValueType.ISO8601
+        decoded, _ = decode_value(encoded, ValueType.ISO8601)
+        assert isinstance(decoded, datetime.datetime)
+        assert decoded.year == 1
+        assert decoded.month == 1
+        assert decoded.day == 1
+
     def test_decode_iso8601_empty_string_returns_none(self) -> None:
         """Empty ISO8601 string should decode as None, matching Go's nil."""
         encoded, _ = encode_value("", ValueType.ISO8601)