Pin Cursor.description NULL-type-code → None mapping at unit level

The cycle 22 mapping replaces ``ValueType.NULL`` (5) with
``None`` in ``description[i].type_code`` so the surface
satisfies PEP 249 §6.1.2 ("type_code must compare equal to
one of the Type Objects" — NULL is not one of the five).

Existing coverage:
- ``test_types_dbapi_compare`` pins the Type-Object surface
  contract (NULL is intentionally exempt from
  ``_DBAPIType``).
- ``test_description_strict_type_codes`` covers the
  mismatched-column-count guard.
- The integration test in ``test_misc_coverage.py``
  exercises the mapping end-to-end.

What was missing: a unit-level pin for the mixed-row case
(some columns NULL-typed, others with real type codes — the
load-bearing path for the comprehension). Add three pins:
mixed-row sync, mixed-row async, all-NULL columns. A
refactor that drops the comprehension surfaces immediately.

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

Author: antoineleclair

tests/test_description_null_type_code_to_none.py

@@ -0,0 +1,106 @@
+"""Pin: ``Cursor.description`` / ``AsyncCursor.description`` map
+``ValueType.NULL`` to ``None`` (cycle 22 mapping).
+
+Wire-layer returns ``column_types`` derived from row 0; if a
+column was tagged ``ValueType.NULL`` (e.g. a ``LEFT JOIN``
+unmatched row, or a literal ``SELECT NULL``), the cycle 22
+mapping replaces ``ValueType.NULL`` (5) with ``None`` in the
+description's ``type_code`` slot. PEP 249 §6.1.2 says the
+type_code "must compare equal to one of the Type Objects" —
+NULL is not one of the five Type Objects, and surfacing
+``None`` matches the documented empty-result-set deviation
+already in this module.
+
+The mixed-row case is the load-bearing surface for the test
+gap: a column tagged NULL alongside columns with real type
+codes. A refactor that drops the comprehension would silently
+re-introduce ``ValueType.NULL`` into description and break
+every cross-driver type-branch.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+from unittest.mock import MagicMock
+
+import pytest
+
+from dqlitedbapi.aio.cursor import AsyncCursor
+from dqlitedbapi.cursor import Cursor
+from dqlitewire.constants import ValueType
+
+
+class _MixedNullTypesClient:
+    """Returns ``column_types=[TEXT, NULL, INTEGER]`` — the
+    mixed-row case the cycle 22 mapping must handle."""
+
+    async def query_raw_typed(
+        self, sql: str, params: Any
+    ) -> tuple[list[str], list[int], list[list[int]], list[list[Any]]]:
+        return (
+            ["name", "n", "age"],
+            [int(ValueType.TEXT), int(ValueType.NULL), int(ValueType.INTEGER)],
+            [[3, 5, 1], [3, 5, 1]],
+            [["alice", None, 30], ["bob", None, 31]],
+        )
+
+
+class _AllNullClient:
+    async def query_raw_typed(
+        self, sql: str, params: Any
+    ) -> tuple[list[str], list[int], list[list[int]], list[list[Any]]]:
+        return (
+            ["a", "b"],
+            [int(ValueType.NULL), int(ValueType.NULL)],
+            [[5, 5]],
+            [[None, None]],
+        )
+
+
+@pytest.mark.asyncio
+async def test_sync_description_maps_null_to_none_in_mixed_row() -> None:
+    conn = MagicMock()
+
+    async def _get() -> object:
+        return _MixedNullTypesClient()
+
+    conn._get_async_connection = _get
+    cur = Cursor(conn)
+    await cur._execute_async("SELECT name, NULL AS n, age FROM users")
+
+    assert cur.description is not None
+    assert cur.description[0][1] == ValueType.TEXT
+    assert cur.description[1][1] is None
+    assert cur.description[2][1] == ValueType.INTEGER
+
+
+@pytest.mark.asyncio
+async def test_async_description_maps_null_to_none_in_mixed_row() -> None:
+    conn = MagicMock()
+
+    async def _ensure() -> object:
+        return _MixedNullTypesClient()
+
+    conn._ensure_connection = _ensure
+    cur = AsyncCursor(conn)
+    await cur._execute_unlocked("SELECT name, NULL AS n, age FROM users", ())
+
+    assert cur.description is not None
+    assert cur.description[0][1] == ValueType.TEXT
+    assert cur.description[1][1] is None
+    assert cur.description[2][1] == ValueType.INTEGER
+
+
+@pytest.mark.asyncio
+async def test_sync_description_all_null_columns_all_none() -> None:
+    conn = MagicMock()
+
+    async def _get() -> object:
+        return _AllNullClient()
+
+    conn._get_async_connection = _get
+    cur = Cursor(conn)
+    await cur._execute_async("SELECT NULL AS a, NULL AS b")
+
+    assert cur.description is not None
+    assert all(d[1] is None for d in cur.description)