Implement Order Modification Feature

You are given an existing C++ trading system with a client/server architecture. The current system already supports two request types: - `AddOrder(order_id, symbol, side, price, quantity)` - `CancelOrder(order_id)` The server keeps all active orders in memory, keyed by `order_id`. Add support for a new request: - `ModifyOrder(order_id, new_price, new_quantity)` Requirements: 1. If the order does not exist or has already been canceled, the modification must be rejected. 2. `symbol` and `side` cannot be changed by a modify request. 3. A successful modification updates only `price` and `quantity` for the existing order. 4. `new_price` and `new_quantity` must both be positive; otherwise reject the request. 5. The order must remain associated with the same `order_id` after modification. 6. Integrate the new request into the existing client/server flow in the same style as the existing add/cancel functionality. Implement the feature and explain what parts of the codebase you would inspect or reuse before making changes.