[Bug]: Partial payment Payment Entry creation uses wrong account currencies in multi-currency setups

[my-dev-jour]

Bug Description

The partial payment flow builds Payment Entry documents manually and sets both paid_from_account_currency and paid_to_account_currency from the invoice currency.

That diverges from ERPNext core Payment Entry logic, where party-account currency and bank/cash-account currency are resolved separately.

Expected Behavior

Partial payment Payment Entry creation should follow ERPNext core currency/account setup for:

• party account currency
• bank/cash account currency
• exchange-rate handling
• reference allocation

Actual Behavior

The custom partial payment path hardcodes account currencies from the invoice currency.

Steps to Reproduce

1. Use POSNext partial payments in a multi-currency environment
2. Create a sale where invoice currency differs from one or more linked accounts
3. Add a partial payment through the POSNext flow
4. Inspect the created Payment Entry values and resulting paid/outstanding behavior

Impact

• incorrect paid/outstanding amounts in multi-currency sites
• wrong exchange-rate or account-currency behavior
• divergence from ERPNext core accounting logic

Suggested Fix

• use ERPNext core get_payment_entry(...) flow where possible
• or mirror ERPNext's account-currency resolution logic exactly instead of deriving both sides from invoice currency

Notes

This is separate from the partial-payment atomicity issue. The problem here is incorrect multi-currency setup even when the transaction itself succeeds.