Get product swap variant groups for a contract

Retrieves product swap variant groups for the next 10 billing cycles of a specific subscription contract. This endpoint calculates and returns which products will be swapped to in future billing cycles based on configured swap automations. <details> <summary>**Full behavior, validation rules, and side effects**</summary> **Response Structure:** Returns a 2D array (List<List<VariantQuantity>>): - **Outer array**: Represents the next 10 billing cycles - **Inner arrays**: Contains the variant(s) that will be swapped to for that specific cycle - **Index 0**: Variants for the next (upcoming) billing cycle - **Index 1**: Variants for the cycle after that - And so on for the next 10 cycles **Variant Details:** Each variant object includes: - **variantId**: Shopify variant ID - **quantity**: Number of units to swap - **title**: Variant title (e.g., "Medium Roast - 12oz") - **image**: Product/variant image URL - **productTitle**: Full product name - **productId**: Shopify product GID - **variantTitle**: Full display name combining product and variant titles - **swapId**: ID of the swap automation rule that triggered this swap **How It Works:** 1. Takes the current products in the subscription 2. Applies configured swap automations for the next 10 cycles 3. Calculates which products will be swapped based on: - Billing cycle number - Swap rule configurations (forBillingCycle, checkForEveryRecurringOrder) - Rule sequence/priority 4. Returns the projected product lineup for each cycle **Use Cases:** - Preview upcoming product swaps in customer portals - Show customers their subscription product timeline - Build interactive swap calendars - Display "what you'll receive" for future orders - Debug and verify swap automation configurations **Important Notes:** - Returns 10 cycles even if no swaps are configured (returns current products) - Multiple variants in an inner array means multiple products will be in that order - Empty inner arrays indicate no products for that cycle (rare edge case) - The swapId can be used to trace which automation rule triggered the swap **Authentication:** Requires valid X-API-Key header </details>