feat: sync sales order if not found during invoice preparation

[devinvento]

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[devinvento]

Fix: Handle Shopify Webhook Race Conditions

This update ensures reliable synchronization between Shopify and ERPNext by handling out-of-order webhook execution.

Problem

Shopify may send orders/paid before orders/create, causing Sales Invoice creation to fail due to a missing Sales Order.

Solution

Added a fallback mechanism in prepare_sales_invoice:

If the Sales Order is not found, call sync_sales_order(order, request_id) to create it immediately.
Re-fetch the Sales Order and proceed with invoice creation.
Benefits
Improved reliability: Prevents "Sales Order not found" errors.
Ensures data integrity: Guarantees every paid order creates both Sales Order and Sales Invoice.
No duplicates: Existing checks prevent duplicate record creation.