Review booking model + migration + factory

[drewroberts]

Comment per @sl0wik:

Make sure that booking migration + factory matching whimsical.

Make sure model booking logic is migrated from previous version.

Comments

This can be replaced with →slot_number checking:

if (empty($booking->slot_id)) {
                throw new \Exception('A booking must be for an availability slot.');
}

All pricing generation should come form checkout packages (ref to generatePricing). Ref: https://github.com/tipoff/support/blob/main/src/Contracts/Checkout/CartInterface.php (check BaseItemContainerInterface )