Fix transaction ID handling in SnappPay gateway

[alissn]

Hi,
This PR fixes the issue of passing the TransactionId into the SnappPay gateway.

Changes made:

• Added a check to see if the order_id key exists in the invoice details
• If order_id is not set, the UUID is converted to an integer using crc32() and set as the new UUID

Why this is needed:
Previously, the gateway was using getTransactionId() directly, which caused issues because the transactionId field is not populated until after the purchase() method is called.

Additional context:
This change synchronizes the SnappPay driver with the implementation used in other payment gateways, ensuring consistent behavior across the codebase.

Fixes: #338