vcr: name gRPC network transport in publish-failure error message (+ backports) #4517

Tracks the third checklist item from #4515 (comment): "Test whether a (helpful) message is returned by the IssueVC REST API operation when issuance over both OpenID4VCI and gRPC network fails. It should describe why both methods failed."

Together with #4470 (joins the OpenID4VCI and network-publish errors), #4516 makes the network-publish error explicitly name gRPC as the transport, so the combined error read by an IssueVC caller distinguishes the two failures instead of showing two near-identical messages.

Tasks

• PR to master: fix(vcr): name gRPC network transport in publish-failure error message #4516
• Backport to v6.2: fix(vcr): clarify network-publish error message names gRPC as the transport (backport to v6.2) #4520
• Backport to v5.4: fix(vcr): clarify network-publish error message names gRPC as the transport (backport to v5.4) #4521

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