Spec Issue: Transfer Termination Message `@type` is required

[jmcanterafonseca-iota]

Bug Report

At
https://eclipse-dataspace-protocol-base.github.io/DataspaceProtocol/2025-1-err1/#transfer-termination-message

@type	OPTIONAL	string	Value must be TransferTerminationMessage

It shall be REQUIRED

[juliapampus]

Ref. #248