fix(signal): pass is_dm to _handle_message so DM pairing flow runs

BaseChannel._handle_message uses is_dm to decide whether to issue a
pairing code when is_allowed rejects the sender. Without it the base
class treats every denied message as a group message and silently
drops it. Forward is_dm=not is_group_message so unapproved DM users
get a pairing code through the standard flow.

This change only takes effect once denied DMs actually reach
_handle_message (next commit); on its own it is a no-op since the
policy gate still short-circuits before this call.

Addresses review comment on PR #3852.

nanobot/channels/signal.py

@@ -732,6 +732,7 @@ class SignalChannel(BaseChannel):
                     "is_group": is_group_message,
                     "group_id": group_id,
                 },
+                is_dm=not is_group_message,
             )
         except Exception:
             await self._stop_typing(chat_id)