Pin _classify_operational dispatch for code=0 and code=None

Upstream's gateway emits FailureResponse(code=0, "empty statement")
on empty / comment-only / no-bound-parameters SQL. The wire layer
accepts the code; the dbapi classifier should route it through to
the default OperationalError (no special PEP 249 mapping). Pin
both the code=0 (empty-statement upstream emission) and the
code=None (wire-decode / ProtocolError-wrapped) dispatches.

Test-only addition; no production code change.

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

Author: antoineleclair

tests/test_classify_operational_code_zero.py

@@ -0,0 +1,29 @@
+"""Pin: ``_classify_operational(0)`` routes ``FailureResponse(code=0,
+"empty statement")`` to a clean ``OperationalError``.
+
+Upstream's gateway emits ``failure(req, 0, "empty statement")`` on
+empty / comment-only SQL; the wire layer accepts the code (after the
+fix). The dbapi classifier dispatches by primary code via the
+``_CODE_TO_EXCEPTION`` table; code 0 falls through to the default
+``OperationalError`` class and is exposed to the user with the
+original message preserved.
+"""
+
+from __future__ import annotations
+
+from dqlitedbapi.cursor import _classify_operational
+from dqlitedbapi.exceptions import OperationalError
+
+
+def test_classify_operational_code_zero_returns_operational_error_class() -> None:
+    """Code 0 is not mapped in _CODE_TO_EXCEPTION; the classifier
+    falls through to the default OperationalError class."""
+    cls = _classify_operational(0)
+    assert cls is OperationalError
+
+
+def test_classify_operational_code_none_returns_operational_error_class() -> None:
+    """Wire-decode / ProtocolError-wrapped errors carry code=None
+    and fall through to OperationalError too."""
+    cls = _classify_operational(None)
+    assert cls is OperationalError