Add unit tests for _iso8601_from_datetime encoder branches

The encoder was previously exercised only by integration tests. Pin
microsecond zero-padding, positive/negative/zero tz offsets, and the
date-vs-datetime fall-through as explicit unit cases so a regression
in any branch (dropped microseconds, mis-signed offset, date routed
through the datetime branch) fails the suite immediately instead of
waiting for a round-trip integration scenario to notice.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

tests/test_iso8601_encoder.py

@@ -0,0 +1,61 @@
+"""Encoder-side tests for ``_iso8601_from_datetime``.
+
+The decode side (``_datetime_from_iso8601``) has explicit error-path
+tests; the encoder was only exercised by integration tests. These unit
+tests pin the microsecond padding, tz-offset sign/magnitude, and the
+``date``-vs-``datetime`` fall-through so a regression in any branch
+surfaces quickly.
+"""
+
+import datetime
+
+from dqlitedbapi.types import _iso8601_from_datetime
+
+
+class TestIso8601FromDatetime:
+    def test_datetime_without_microseconds(self) -> None:
+        d = datetime.datetime(2025, 1, 1, 12, 0, 0)
+        assert _iso8601_from_datetime(d) == "2025-01-01 12:00:00"
+
+    def test_datetime_with_microseconds_zero_padded(self) -> None:
+        d = datetime.datetime(2025, 1, 1, 12, 0, 0, microsecond=7)
+        # Six-digit padded microseconds.
+        assert _iso8601_from_datetime(d) == "2025-01-01 12:00:00.000007"
+
+    def test_datetime_with_six_digit_microseconds(self) -> None:
+        d = datetime.datetime(2025, 1, 1, 12, 0, 0, microsecond=999999)
+        assert _iso8601_from_datetime(d) == "2025-01-01 12:00:00.999999"
+
+    def test_naive_datetime_has_no_offset(self) -> None:
+        """Naive datetimes emit the bare ISO string (no trailing offset)."""
+        d = datetime.datetime(2025, 1, 1, 12, 0, 0)
+        result = _iso8601_from_datetime(d)
+        assert "+" not in result and result.count("-") == 2
+
+    def test_utc_offset_emitted_as_plus_zero(self) -> None:
+        d = datetime.datetime(2025, 1, 1, 12, 0, 0, tzinfo=datetime.UTC)
+        result = _iso8601_from_datetime(d)
+        assert result.endswith("+00:00")
+
+    def test_positive_offset(self) -> None:
+        tz = datetime.timezone(datetime.timedelta(hours=5, minutes=30))
+        d = datetime.datetime(2025, 1, 1, 12, 0, 0, tzinfo=tz)
+        assert _iso8601_from_datetime(d).endswith("+05:30")
+
+    def test_negative_offset(self) -> None:
+        tz = datetime.timezone(datetime.timedelta(hours=-5, minutes=-30))
+        d = datetime.datetime(2025, 1, 1, 12, 0, 0, tzinfo=tz)
+        assert _iso8601_from_datetime(d).endswith("-05:30")
+
+    def test_negative_offset_with_microseconds(self) -> None:
+        tz = datetime.timezone(datetime.timedelta(hours=-8))
+        d = datetime.datetime(2025, 6, 15, 9, 30, 45, microsecond=42, tzinfo=tz)
+        assert _iso8601_from_datetime(d) == "2025-06-15 09:30:45.000042-08:00"
+
+    def test_date_only_takes_fall_through_branch(self) -> None:
+        """``date`` (not ``datetime``) must produce the short YYYY-MM-DD
+        form via the ``isoformat()`` fall-through, not the datetime
+        branch that would call strftime with time components.
+        """
+        d = datetime.date(2025, 1, 1)
+        assert _iso8601_from_datetime(d) == "2025-01-01"