fix: Parse protobuf encoded keys for contract

[manishdait]

While debugging the logs in https://github.com/OpenElements/hiero-enterprise-java/actions/runs/22081032269/job/63806078906?pr=224#step:7:538

failure is happening in MirrorNodeJsonConverterImpl.toContract() at:

PublicKey adminKey =
    node.get("admin_key").isNull()
        ? null
        : PublicKey.fromString(node.get("admin_key").get("key").asText());

// while mirror node response retrns:
"admin_key": {
  "_type": "ProtobufEncoded",
  "key": "0a03189908"
}

_type = "ProtobufEncoded" is a serialized protobuf Key message. however, PublicKey.fromString() expects DER / raw Ed25519 / raw ECDSA formats. Passing protobuf serialized bytes causes parsing to fail internally causes the stacktrace.

This PR fix the parsing of admin_key in toContract() for ProtobufEncoded key types.

[manishdait]

closing as repo get migrated