test: cover DM session key isolation

Update interrupt-key expectations for namespaced DM session keys and add a regression test that different DM chat IDs produce distinct gateway sessions.

tests/gateway/test_interrupt_key_match.py

@@ -50,11 +50,11 @@ class TestInterruptKeyConsistency:
     """Ensure adapter interrupt methods are queried with session_key, not chat_id."""
 
     def test_session_key_differs_from_chat_id_for_dm(self):
-        """Session key for a DM is NOT the same as chat_id."""
+        """Session key for a DM is namespaced and includes the DM chat_id."""
         source = _source("123456", "dm")
         session_key = build_session_key(source)
         assert session_key != source.chat_id
-        assert session_key == "agent:main:telegram:dm"
+        assert session_key == "agent:main:telegram:dm:123456"
 
     def test_session_key_differs_from_chat_id_for_group(self):
         """Session key for a group chat includes prefix, unlike raw chat_id."""

tests/gateway/test_session.py

@@ -379,6 +379,15 @@ class TestWhatsAppDMSessionKeyConsistency:
         key = build_session_key(source)
         assert key == "agent:main:telegram:dm:99"
 
+    def test_distinct_dm_chat_ids_get_distinct_session_keys(self):
+        """Different DM chats must not collapse into one shared session."""
+        first = SessionSource(platform=Platform.TELEGRAM, chat_id="99", chat_type="dm")
+        second = SessionSource(platform=Platform.TELEGRAM, chat_id="100", chat_type="dm")
+
+        assert build_session_key(first) == "agent:main:telegram:dm:99"
+        assert build_session_key(second) == "agent:main:telegram:dm:100"
+        assert build_session_key(first) != build_session_key(second)
+
     def test_discord_group_includes_chat_id(self):
         """Group/channel keys include chat_type and chat_id."""
         source = SessionSource(