Spec issue: Offer `@type` should be mandatory.

[jmcanterafonseca-iota]

At https://eclipse-dataspace-protocol-base.github.io/DataspaceProtocol/2025-1/#Offer-table it incorrectly says that @type is optional and it should say is mandatory.

[juliapampus]

Committer round 15.01.2026: @type should be mandatory or non-existing.