Fix: Support zero-payment private listings

[ryanio]

Summary

Fixes SDK crash when fulfilling zero-payment private listings (e.g., rewards claims).

Closes #1832 - This PR supersedes #1832 by properly supporting zero-payment private listings instead of just throwing an error.

Problem

The SDK crashes when fulfilling private listings with no payment consideration items:

1. constructPrivateListingCounterOrder accesses paymentItems[0] without checking if the array is empty
2. fulfillPrivateOrder accesses counterOrder.parameters.offer[0].startAmount which is undefined for zero-payment listings

Why #1832's approach was insufficient

PR #1832 proposed throwing an error when paymentItems is empty. However, investigation of the backend (os2-core) revealed that zero-payment private listings are valid and deliberately supported:

• PrivateListingValidationRule.kt bypasses the pricePerItem <= 0 check for REWARDS_PRIVATE_LISTING_CLAIM_WALLET
• SeaportListingValidationRules.kt bypasses the "must have 1-7 currency items" check for the same wallet
• Seaport.kt:1563-1574 explicitly handles empty payment items by returning offer: emptyList()

The backend already handles this correctly - the SDK should too.

Solution

Mirror the backend behavior:

1. Allow empty payment items - constructPrivateListingCounterOrder now returns a counter order with offer: [] for zero-payment listings instead of crashing

2. Compute value from original order - Added computePrivateListingValue() that sums native currency consideration items from the original order, handling both standard and zero-payment cases

3. Use computed value - fulfillPrivateOrder now uses computePrivateListingValue() instead of counterOrder.parameters.offer[0].startAmount

Changes

• src/orders/privateListings.ts:

  • Added computePrivateListingValue() helper function
  • Updated constructPrivateListingCounterOrder() to handle empty payment items

• src/sdk/fulfillment.ts:

  • Import and use computePrivateListingValue()
  • Calculate ETH value from original order instead of counter order

Testing

• All existing tests pass
• Lint passes
• Type check passes