Fix: Improve inventory validation and handle empty cart/inventory

[seer-by-sentry]

👋 Hi there! This PR was automatically generated by Autofix 🤖

This fix was triggered by Neil Manvar.

Fixes FLASK-1N. The issue was that: Requested quantity (6) for product 3 exceeded available inventory (1), triggering the 'Not enough inventory' exception during checkout.

• Ensure product IDs are strings when formatting for the SQL query.
• Refactor inventory validation to use a map for efficient lookup.
• Improve error messages to include product IDs and quantities.
• Add checks for empty cart or inventory to prevent errors.
• Increment checkout.failed metric when inventory validation fails.

If you have any questions or feedback for the Sentry team about this fix, please email [email protected] with the Run ID: 38736.

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[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: flask/src/db.py

Function	Unhandled Issue
get_inventory	DatabaseConnectionError: get_inventory checkout Event Count: 38

📄 File: flask/src/main.py (Click to Expand)

Function	Unhandled Issue
checkout	Exception: Not enough inventory for product checkout Event Count: 4.9k
checkout	DatabaseConnectionError: get_inventory checkout Event Count: 38
checkout	UnboundLocalError: cannot access local variable 'e' where it is not associated with a value ... Event Count: 1

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