Fix CI failures on root — mock OSError instead of relying on filesystem

Tests assumed write to /nonexistent/... fails, but CI runs as root where
Linux allows creating any path. Use unittest.mock.patch on builtins.open
with OSError side_effect so the test is environment-independent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_firewall_manager.py

@@ -133,7 +133,8 @@ class TestGenerateCorefile(unittest.TestCase):
         self.assertIn('reload', content)
 
     def test_returns_false_on_write_error(self):
-        result = firewall_manager.generate_corefile([], '/nonexistent/path/Corefile')
+        with unittest.mock.patch('builtins.open', side_effect=OSError('Permission denied')):
+            result = firewall_manager.generate_corefile([], '/any/path/Corefile')
         self.assertFalse(result)