Improve checkout process and error handling

[seer-by-sentry]

Fixes REACT-5FV. The issue was that: Python 2 print syntax in process_order causes SyntaxError, preventing inventory validation and leading to a 500 Internal Server Error.

• Added deep copy to avoid modifying the global Inventory directly.
• Implemented more robust error handling for missing or invalid cart items.
• Added error handling for missing 'cart' in request body.
• Improved logging messages for clarity.
• Added Sentry error capture for exceptions during checkout.
• Return error codes to the client.

This fix was generated by Seer in Sentry, triggered by [email protected]. 👁️ Run ID: 311391

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