Implement inventory management and error handling for checkout

[seer-by-sentry]

Fixes REACT-5YD. The issue was that: Backend inventory check failed due to insufficient stock, raising an unhandled exception that resulted in a 500 Internal Server Error response.

• Added InsufficientInventoryError exception to handle cases where the requested quantity exceeds available stock.
• Modified process_order to check for sufficient inventory before processing the order and raise InsufficientInventoryError if needed.
• Updated the /checkout route to handle InsufficientInventoryError and return a 409 status code with details about the error.
• Added more robust error handling to the /checkout route to catch general exceptions and return a 500 status code.
• Modified the /checkout route to correctly parse the cart items and quantities from the request data.
• Added copy() to the Inventory dictionary to prevent modification of the global Inventory variable.

This fix was generated by Seer in Sentry, triggered by [email protected]. 👁️ Run ID: 98037

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