docs(pairing): address reviewer comments — comments, error msg, __all__ test

- Clarify SystemExit message for missing/null allowFrom (manager.py)
- Document why Feishu passes content="" for unauthorized DMs
- Document exact-match semantics in BaseChannel.is_allowed()
- Document negligible collision probability in generate_code()
- Add test_all_exports_are_importable for nanobot.pairing.__all__

nanobot/channels/base.py

@@ -202,6 +202,7 @@ class BaseChannel(ABC):
             allow_list = getattr(self.config, "allow_from", []) or []
         if "*" in allow_list:
             return True
+        # allowFrom entries are opaque tokens — must match exactly.
         if str(sender_id) in allow_list:
             return True
         if is_approved(self.name, str(sender_id)):

nanobot/channels/feishu.py

@@ -1716,6 +1716,8 @@ class FeishuChannel(BaseChannel):
             # Group chats are silently ignored; DMs get a pairing code.
             if not self.is_allowed(sender_id):
                 if chat_type == "p2p":
+                    # content="" because the pairing reply is generated by
+                    # BaseChannel._handle_message, not from the original message.
                     await self._handle_message(
                         sender_id=sender_id,
                         chat_id=sender_id,

nanobot/channels/manager.py

@@ -145,7 +145,7 @@ class ChannelManager:
                 allow = getattr(cfg, "allow_from", None)
             if allow is None:
                 raise SystemExit(
-                    f'Error: "{name}" is missing allowFrom. '
+                    f'Error: "{name}" is missing or null allowFrom. '
                     f'Set ["*"] to allow everyone, or add specific user IDs.'
                 )