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/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.'
                 )