Pydantic Base64Bytes silently corrupts raw bytes passed without encoding

Base64Bytes validates input by attempting to base64-decode it. Passing raw binary bytes (e.g. b"\x89PNG...") does not raise a validation error, Pydantic interprets the raw bytes as base64 characters and produces silently corrupted output. This makes the bug invisible until downstream consumers read garbage data. Only base64.b64encode(raw_bytes) or a base64-encoded string should be passed at construction time.

Always base64.b64encode() raw bytes before assigning to a Base64Bytes field. Add a JSON round-trip assertion in tests to catch corruption early.