fix(gateway/telegram): fall back to chat.id when from_user is None in DMs

When `message.from_user` is None — which can happen for forwarded messages,
anonymous admin mode in groups, or certain Telegram client edge cases —
`_build_message_event` set `source.user_id` to None. This caused:

1. `_is_user_authorized()` to early-return False (`if not user_id: return False`)
2. The access check never compared against `TELEGRAM_ALLOWED_USERS` even when
   the user actually was in the allowlist
3. The pairing flow fired and generated a code for `user_id=None`
4. The pairing approval saved an entry under the literal string key "null"
5. The user was effectively locked out because their real user_id never
   matched the "null" key on subsequent messages

For DMs (`chat_type == "dm"`), Telegram guarantees `chat.id == user.id` —
they are the same numeric ID for private chats. Falling back to `chat.id`
when `from_user` is None for DMs restores the expected access-control
behavior without weakening it (group/channel chats correctly stay None).

Also adds a parallel `user_name` fallback to `chat.full_name` so the
display name still works in the same edge case.

gateway/platforms/telegram.py

@@ -2926,8 +2926,8 @@ class TelegramAdapter(BasePlatformAdapter):
             chat_id=str(chat.id),
             chat_name=chat.title or (chat.full_name if hasattr(chat, "full_name") else None),
             chat_type=chat_type,
-            user_id=str(user.id) if user else None,
-            user_name=user.full_name if user else None,
+            user_id=str(user.id) if user else (str(chat.id) if chat_type == "dm" else None),
+            user_name=user.full_name if user else (chat.full_name if hasattr(chat, "full_name") and chat_type == "dm" else None),
             thread_id=thread_id_str,
             chat_topic=chat_topic,
         )