feat: add protobuf schema bridge contracts

crates/mcpaas-proto/src/errors.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Clone, Debug, PartialEq, Eq, Error)]
+pub enum ProtoError {
+    #[error("oneof {oneof_name} must contain at least one variant")]
+    EmptyOneof { oneof_name: String },
+}