Refactor checkout and sample download handlers for improved functionality

Commit notes

- Moved AI pack purchase validation to a more appropriate location in the CreateCheckoutSession method. - Enhanced error handling for missing PaymentIntent in the handleCheckoutCompleted method. - Updated download logic in GuestDownloadSample to use a buffered read for improved performance and error handling. - Introduced a new utility function, SanitizeFilename, to ensure safe filename generation for downloads. - Corrected URL generation in the GetPreviewProxyURL method to include a leading slash.

These changes enhance the robustness and clarity of the checkout and sample download processes.