Challenge 1: Write a Service Definition — Possible Solution
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message GetOrderRequest {
  int32 id = 1;
}

service OrderService {
  rpc GetOrder(GetOrderRequest) returns (Order);
}

WHY THIS WORKS AS AN ANSWER
------------------------------
A dedicated GetOrderRequest message is defined to hold the input,
following the exact pattern this chapter's own GetProductRequest
example used — a request message wrapping the single id field, rather
than passing a bare scalar directly, since rpc methods always take a
message type as input, never a raw scalar.

The service block groups this one method under OrderService, using
the rpc MethodName(Request) returns (Response); syntax — GetOrder
takes a GetOrderRequest and returns Order (the message already defined
in Chapter 2), giving the method a clear, unary (one request, one
response) shape matching what was asked for.