Challenge 2: Add a Nested Message — Possible Solution
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message ShippingInfo {
  string address = 1;
  bool expedited = 2;
}

message Order {
  int32 id = 1;
  string customer_name = 2;
  repeated string item_names = 3;
  ShippingInfo shipping_info = 4;
}

WHY THIS WORKS AS AN ANSWER
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ShippingInfo is defined as its OWN separate message, with its own
independent field numbering starting at 1 — nested messages have
their own numbering scope entirely separate from whatever message
uses them, exactly like Address had its own 1/2 numbering independent
of Customer's 1/2/3 in this chapter's own nested-message example.

Order then references ShippingInfo simply by using it as a field
TYPE (ShippingInfo shipping_info = 4;), the same way Customer used
Address as a field type — this is what "nesting" actually means in
proto3: one message type embedded as a field inside another, producing
the same hierarchical shape a nested JSON object would.

shipping_info is assigned field number 4, continuing on from Challenge
1's existing fields (1, 2, 3) without reusing or conflicting with any
of them.