fix: store session bytes as base64 to avoid invalid encoding errors

[hucruz]

fix: store session bytes as base64 to avoid encoding errors

Previously, session fields of type bytes were serialized as UTF-8 strings or
Python literal representations (e.g., b'...'), causing validation errors
(bytes_invalid_encoding) when attempting to restore the data.

This change ensures that all byte values are encoded in base64 when saving, guaranteeing compatibility with Pydantic and
other session consumers.