AMMVote: `applyVote` writes `sfDiscountedFee` to expired auction slot

[mvadari]

Issue Description

In AMMVote::applyVote (src/libxrpl/tx/transactors/dex/AMMVote.cpp, lines 181–203), the sfDiscountedFee field on the auction slot is unconditionally updated without checking whether the auction slot has expired:

if (ammSle->isFieldPresent(sfAuctionSlot))
{
    auto& auctionSlot = ammSle->peekFieldObject(sfAuctionSlot);
    if (auto const discountedFee = fee / AUCTION_SLOT_DISCOUNTED_FEE_FRACTION)
    {
        auctionSlot.setFieldU16(sfDiscountedFee, discountedFee);
    }
    // ...
}

The read side (getTradingFee in AMMUtils.cpp) correctly checks the auction slot's sfExpiration before applying the discounted fee. So the stale write is never actually used during swaps. However, it is unnecessary state pollution — writing a field that will never be read until the slot is re-bid.

Steps to Reproduce

1. Create an AMM with an auction slot.
2. Let the auction slot expire.
3. Submit an AMMVote transaction.
4. Observe that sfDiscountedFee is written to the expired auction slot.

Expected Result

applyVote should check the auction slot's sfExpiration before writing sfDiscountedFee. If the slot is expired, the discounted fee write should be skipped.

Actual Result

sfDiscountedFee is unconditionally written to the auction slot, even when expired.

Environment

All versions with AMM support (XLS-30).

Supporting Files

N/A