getattr() on a dict never finds custom keys — it only searches
object attributes, not dict keys. When channel config is loaded as
a Pydantic extra field (which is a plain dict), getattr(config,
'allow_from', []) always returns the default [], causing all access
to be denied regardless of the allowFrom configuration.
Fix both is_allowed() and _validate_allow_from() to use isinstance
checks, falling back to dict.get() for dict configs while preserving
getattr() for object-style configs.
