test: add coverage for from_user=None DM fallback

Tests the three cases:
- DM with from_user=None: user_id falls back to chat.id
- Group with from_user=None: user_id stays None (safe default)
- DM with from_user present: user_id uses from_user.id (no regression)

tests/gateway/test_dm_topics.py

@@ -645,3 +645,54 @@ def test_group_topic_chat_id_int_string_coercion():
 
     assert event.auto_skill == "hermes-agent-dev"
     assert event.source.chat_topic == "Dev"
+
+
+# ── _build_message_event: from_user=None fallback in DMs ──
+
+
+def test_build_message_event_dm_from_user_none_falls_back_to_chat_id():
+    """When from_user is None in a DM, user_id should fall back to chat.id."""
+    from gateway.platforms.base import MessageType
+
+    adapter = _make_adapter()
+    msg = _make_mock_message(chat_id=12345, user_id=42, user_name="Alice")
+    # Simulate from_user being None (edge case on fresh restart / forwarded msg)
+    msg.from_user = None
+
+    event = adapter._build_message_event(msg, MessageType.TEXT)
+
+    # Should fall back to chat.id since chat_type is "dm"
+    assert event.source.user_id == "12345"
+    assert event.source.user_name == "Alice"  # falls back to chat.full_name
+
+
+def test_build_message_event_group_from_user_none_stays_none():
+    """When from_user is None in a group, user_id should remain None."""
+    from gateway.platforms.base import MessageType
+
+    adapter = _make_adapter()
+    msg = _make_mock_message(
+        chat_id=-1001234567890, chat_type=_ChatType.SUPERGROUP,
+        user_id=42, user_name="Alice"
+    )
+    msg.from_user = None
+
+    event = adapter._build_message_event(msg, MessageType.TEXT)
+
+    # Groups should NOT fall back — anonymous senders stay None
+    assert event.source.user_id is None
+    assert event.source.user_name is None
+
+
+def test_build_message_event_dm_from_user_present_uses_user():
+    """When from_user is present in a DM, it should be used (no fallback)."""
+    from gateway.platforms.base import MessageType
+
+    adapter = _make_adapter()
+    msg = _make_mock_message(chat_id=12345, user_id=99999, user_name="Bob")
+
+    event = adapter._build_message_event(msg, MessageType.TEXT)
+
+    # Normal case — from_user is used directly
+    assert event.source.user_id == "99999"
+    assert event.source.user_name == "Bob"