Assert fetchmany clears the cursor messages at the call boundary

The test appended a sentinel to cur.messages but never checked it was cleared,
so removing the prelude clear would not have failed it. Assert the sentinel is
gone and messages is empty after the call.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: antoineleclair

tests/test_fetchmany_one_clear_per_call_pep249.py

@@ -19,7 +19,8 @@ def test_sync_fetchmany_clears_messages_only_once() -> None:
         cur.messages.append(("Warning", "sentinel"))  # type: ignore[arg-type]
         more = cur.fetchmany(2)
         assert len(more) == 2
-        # The sentinel is wiped by the prelude clear (correct); the real pin is structural below.
+        assert ("Warning", "sentinel") not in cur.messages
+        assert cur.messages == []
     finally:
         conn.close()