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__

tests/pairing/test_store.py

@@ -2,9 +2,18 @@ import time
 
 import pytest
 
+from nanobot.pairing import __all__ as pairing_all
 from nanobot.pairing import store
 
 
+def test_all_exports_are_importable():
+    """Every name in __all__ must actually be importable from nanobot.pairing."""
+    import nanobot.pairing as pkg
+
+    for name in pairing_all:
+        assert hasattr(pkg, name), f"{name} is in __all__ but not exported"
+
+
 @pytest.fixture(autouse=True)
 def _tmp_store(tmp_path, monkeypatch):
     path = tmp_path / "pairing.json"