Marshaling error when getting attestations with inline HELM_CHART materials

[Piskoo]

When getting an attestation that contains a HELM_CHART material stored inline, the gRPC endpoint fails with:

2026-01-22T17:19:55.591+0100	INFO	{"kind": "server", "component": "grpc", "operation": "/controlplane.v1.WorkflowRunService/View", "args": "digest:\"sha256:70b21676da4afc017f858029d083e9c09b30eceabf9c196de1d3c4402a34c52c\"", "code": 0, "reason": "", "stack": "", "latency": 0.112403519}
2026/01/22 17:19:55 ERROR: [core] [Server #1] grpc: server failed to encode response: rpc error: code = Internal desc = grpc: error while marshaling: string field contains invalid UTF-8

To reproduce

chainloop att init --project project --workflow workflow

chainloop att add --kind HELM_CHART --value chart.tgz

chainloop att push

chainloop wf run describe --digest digest

[migmartri]

This is a duplicate of #2065