[AHM] Add account translation to treasury spend beneficiaries during RC->AH migration

[sigurpol]

🎯 Core Issue

The beneficiary field in treasury spends can contain XCM Location structures with AccountId32 junctions that reference specific account IDs. These AccountIds need to be translated from RC format to AH format

#807 has introduced a mocked implemenation in system-parachains/asset-hubs/asset-hub-polkadot/src/ah_migration/mod.rs of translate_location_account_ids() which just returns the same location.

The proper final implementation needs to apply the correct account translation to AccountId32 junctions instead, see details later on.

It's also important to remember to enforce the requirement for idempotency as stated in #808.

See this thread.

Implementation details

Current flow

1. RC Side: Sends RcTreasuryMessage::Spends with beneficiary as VersionedLocation. The translate_location_account_ids introduced by [AHM / ah-migrator]: Implement RC->AH account translation mocked migration across all involved pallets #807 is just a no-op for the time being
2. AH Side: Processes in do_process_treasury_message() using T::RcToAhTreasurySpend::convert()
3. Conversion: RcToAhTreasurySpend::convert() transforms:
   • (VersionedLocatableAsset, VersionedLocation)
   • → (VersionedLocatableAsset, VersionedLocatableAccount)

Two Implementation Options

Option 1: Translate in Treasury.rs (Pallet Level)

Location: pallets/ah-migrator/src/treasury.rs

Approach: Add account translation in do_process_treasury_message() before calling T::RcToAhTreasurySpend::convert().

RcTreasuryMessage::Spends { id: spend_index, status: spend } => {
    // Extract beneficiary and apply account translation
    let translated_beneficiary = Self::translate_beneficiary_location(spend.beneficiary);
    
    // Create updated spend with translated beneficiary
    let updated_spend = treasury_alias::SpendStatus {
        asset_kind: spend.asset_kind,
        amount: spend.amount,
        beneficiary: translated_beneficiary,
        valid_from: spend.valid_from,
        expire_at: spend.expire_at,
        status: spend.status,
    };
    
    // Apply high-level conversion
    let (asset_kind, beneficiary) = T::RcToAhTreasurySpend::convert((
        updated_spend.asset_kind, 
        updated_spend.beneficiary
    ))?;
    
    // Store result...
}

Option 2: Translate in mod.rs (Runtime Level)

Location: system-parachains/asset-hubs/asset-hub-polkadot/src/ah_migration/mod.rs

Approach: Modify mocked translate_location_account_ids() to apply the proper translation

Both options are fine.

[sigurpol]

cc @ggwpez , @muharem. Needed for AHM.

[kianenigma]

Probably already fixed?

runtimes/pallets/ah-migrator/src/xcm_translation.rs

Line 56 in 18b3bf2

pub fn translate_beneficiary_location(

[sigurpol]

Probably already fixed?

runtimes/pallets/ah-migrator/src/xcm_translation.rs

Line 56 in 18b3bf2
pub fn translate_beneficiary_location(

I believe we are still relying on what I did initially on #807 so only check X1(AccountId32) pattern otherwise we return location unchanged. But I haven't spent more time on it after that.
@muharem , @ggwpez can you confirm that we are ok with the current implementation and we can close #this issue?