Previously _validate_allow_from raised SystemExit when allowFrom was
missing, forcing every channel to declare an explicit allowlist.
With the pairing feature this is no longer necessary: a channel with
no allowFrom simply operates in pairing-only mode, letting users
approve senders via /pairing approve <code> from the WebUI or CLI.
- Replace SystemExit with an info log in _validate_allow_from
- Add test_validate_allow_from_allows_missing_allow_from