Optional codec for application state at the Session checkpoint boundary.

Configure shared_state_codec: MyCodec on Session.start_link/1. The module is supplied by the host, never by stored data. encode/1 must return {:ok, json_data}; decode/1 receives JSON data only after the complete snapshot and roster have been validated and returns {:ok, application_state}. Callbacks must be pure and must not include credentials in the persisted data. The codec module and application structs are never inferred from a snapshot. Callback failures return generic errors without exposing the supplied state. Successful decode still precedes the policy's separate restore validation. Without a codec, the existing JSON representation remains unchanged.