Convert executed_fee_amount to surplus token

[sunce86]

Description

As part of the initiative to show all executed order fees in surplus tokens, we have to convert the executed_fee_amount from sell token to surplus token. This is needed by frontend and other integrators, so that ALL fees are in surplus token and easily summarized.

As a reminder, executed_fee_amount represents executed version of a fixed fee_amount fee that is part of the signed order and it's always assumed that it is expressed in sell token. But we can easily convert the EXECUTED REPRESENTATION of this fee into surplus token by following this math:

let executedFeeAmountInSurplusToken = match order.kind {
    Buy => executedFeeAmount,
    Sell => executedFeeAmount * executedBuyAmount / executedSellAmountBeforeFees,
};

This math is 100% equal to using uniform clearing prices to convert between fee in sell and buy tokens, as already used in the existing code:

https://github.com/cowprotocol/services/blob/main/crates/autopilot/src/domain/settlement/trade/math.rs#L126-L129 :

/// Converts given surplus fee into sell token fee.
    fn fee_into_sell_token(&self, fee: eth::TokenAmount) -> Result<eth::SellTokenAmount, Error> {
        let fee_in_sell_token = match self.side {
            order::Side::Buy => fee,
            order::Side::Sell => fee
                .checked_mul(&self.prices.uniform.buy.into())
                .ok_or(error::Math::Overflow)?
                .checked_div(&self.prices.uniform.sell.into())
                .ok_or(error::Math::DivisionByZero)?,
        }
        .into();
        Ok(fee_in_sell_token)
    }

Note 1: This PR only changes how the executed version of this fee is represented over the API. It does not change anything in the database, so it also does not change our assumption of how the fee_amount is saved internally.

Note 2: Only > ~2 year old orders are affected by this. All newer orders have executed_fee_amount = 0.

Note 3: These old orders will have a bad total fee shown when this PR is merged. We need #3184 merged as well (and all consequent data migrations) so that all fees are finally shown in surplus tokens.

Changes

• Executed version of fee_amount is now shown in surplus token

How to test

Confirmed on several examples of orders that have fee_amount > 0.

[github-actions]

Reminder: Please consider backward compatibility when modifying the API specification.
If breaking changes are unavoidable, ensure:

• You explicitly pointed out breaking changes.
• You communicate the changes to affected teams.
• You provide proper versioning and migration mechanisms.

---

Caused by:

• crates/orderbook/openapi.yml

[squadgazzz]

The change looks trivial, but I don't have a full picture of these fee migrations and PRs ordering, so I can't think of any implications atm. I need to go over other PRs again.