`SwapFirstAssetTrader::buy_weight` creates 0-amount AssetsInHolding entry, causing Fungible(0) decode failure in AssetsTrapped event

[clangenb]

When PayFees instruction contains the exact fee amount quoted by query_weight_to_asset_fee, the swap in SwapFirstAssetTrader::buy_weight produces zero change. This zero-amount credit is unconditionally wrapped into an AssetsInHolding entry:

 let unspent = AssetsInHolding::new_from_fungible_credit(id, Box::new(credit_change));
 payment.subsume_assets(unspent);

This 0-amount entry propagates:

1. buy_weight returns it as unspent → PayFees handler puts it in self.fees
2. refund_surplus() moves leftover fees into self.holding
3. post_process() calls drop_assets() because holding appears non-empty
4. drop_assets() emits AssetsTrapped event containing Fungible(0)
5. Fungibility::decode() rejects Fungible(0): "Fungible asset of zero amount is not allowed"

Fix: Guard buy_weight against 0-amount change, matching the existing pattern in refund_weight:

 if credit_change.peek() > 0 {
     let unspent = AssetsInHolding::new_from_fungible_credit(id, Box::new(credit_change));
     payment.subsume_assets(unspent);
 }

Discovered in: PR #10726, commit a2740c6