Upstream AgentApi responses can return attachment objects that don't
implement the Attachment dataclass. _to_core_attachments coerces them
via duck-typing so OutgoingMessage always carries typed Attachment
instances regardless of the upstream response shape.
platform/ shadowed Python's stdlib platform module, breaking
aiogram/aiohttp/multidict at import time. Renamed to sdk/ and
updated all imports across core/, tests/, and adapter/telegram/.