Pin cursor behavior after external connection close

Previously tests covered operations on an explicitly-closed cursor but
not on a live cursor whose connection was closed externally (via
conn.close()). Both the sync and async cursor must raise
InterfaceError on execute / executemany in this state, and cursor.close()
must remain idempotent.

Author: antoineleclair

tests/test_cycle3_tests_pinning.py

@@ -0,0 +1,54 @@
+"""Cycle-3 bundle E: regression tests pinning previously-untested edges.
+
+Covers:
+- ISSUE-103: cursor.execute() on a cursor whose connection was closed
+  externally (not via cursor.close()) raises InterfaceError.
+"""
+
+import asyncio
+
+import pytest
+
+from dqlitedbapi.aio.connection import AsyncConnection
+from dqlitedbapi.aio.cursor import AsyncCursor
+from dqlitedbapi.exceptions import InterfaceError
+
+
+class TestCursorAfterExternalConnectionClose:
+    """ISSUE-103: operations on a live cursor whose connection was
+    closed externally must raise InterfaceError rather than hanging
+    or surfacing a cryptic lower-layer error.
+    """
+
+    def test_sync_cursor_execute_after_connection_close(self) -> None:
+        import dqlitedbapi
+
+        conn = dqlitedbapi.connect("localhost:19001")
+        cursor = conn.cursor()
+        conn.close()
+
+        with pytest.raises(InterfaceError):
+            cursor.execute("SELECT 1")
+
+        with pytest.raises(InterfaceError):
+            cursor.executemany("SELECT ?", [(1,), (2,)])
+
+        # cursor.close() remains idempotent even after the connection is gone.
+        cursor.close()
+        cursor.close()
+
+    def test_async_cursor_execute_after_connection_close(self) -> None:
+        async def _run() -> None:
+            conn = AsyncConnection("localhost:19001")
+            # Don't actually connect — keep this a pure state-machine test
+            # so it doesn't depend on the cluster being up.
+            cursor = AsyncCursor(conn)
+            await conn.close()
+
+            with pytest.raises(InterfaceError):
+                await cursor.execute("SELECT 1")
+
+            with pytest.raises(InterfaceError):
+                await cursor.executemany("SELECT ?", [(1,), (2,)])
+
+        asyncio.run(_run())