S14: NPE fail-closed adapter; npe mode never falls back to lab-xor

tests/test_sm.py

@@ -33,6 +33,15 @@ class SignedConfigTests(unittest.TestCase):
                 load_signed(p, pub)
 
 
+class NpeAdapterTests(unittest.TestCase):
+    def test_npe_mode_fail_closed(self):
+        from secure_messaging.npe_adapter import NpeRequired
+        from secure_messaging.envelope import seal
+
+        with self.assertRaises(NpeRequired):
+            seal(to="npe.inbox.x", sender="alice", body={"a": 1}, mode="npe")
+
+
 class EnvelopeTests(unittest.TestCase):
     def test_lab_xor_roundtrip_lookup(self):
         env = seal(