Fix update-bank-transaction 400 when editing line items

[pedromachados]

Problem

Calling update-bank-transaction to edit an existing bank transaction's line items fails with:

HTTP 400 — "The document sub total does not equal the sub total of the lines"

This happens even when only a line description changes and the monetary amount is identical.

Root cause

updateBankTransaction in src/handlers/update-xero-bank-transaction.handler.ts builds the update payload by spreading the entire fetched transaction:

const bankTransaction: BankTransaction = {
  ...existingBankTransaction,   // carries read-only computed subTotal/total/totalTax
  ...
  lineItems: lineItems ? lineItems : existingBankTransaction.lineItems,
  ...
};

The spread carries Xero's read-only computed fields — subTotal, total, totalTax. When lineItems are replaced, Xero recomputes the line totals server-side, but the stale subTotal/total copied from the fetched object no longer reconcile, so the API rejects the update.

Per the Xero BankTransactions API docs, SubTotal, Total, and TotalTax are each "calculated by Xero and cannot be overridden."

Fix

Build a minimal payload containing only the editable fields plus the attributes Xero needs on update, and omit the computed totals so Xero recomputes them:

const bankTransaction: BankTransaction = {
  bankTransactionID: bankTransactionId,
  bankAccount: existingBankTransaction.bankAccount,
  lineAmountTypes: existingBankTransaction.lineAmountTypes,
  status: existingBankTransaction.status,
  currencyCode: existingBankTransaction.currencyCode,
  type: type ? BankTransaction.TypeEnum[type] : existingBankTransaction.type,
  contact: contactId ? { contactID: contactId } : existingBankTransaction.contact,
  lineItems: lineItems ? lineItems : existingBankTransaction.lineItems,
  reference: reference ? reference : existingBankTransaction.reference,
  date: date ? date : existingBankTransaction.date,
};

lineAmountTypes is preserved from the existing transaction rather than omitted: it defaults to Exclusive when absent, so dropping it could silently change the tax treatment (and therefore the recomputed totals) on inclusive-tax transactions.

Verification

• npm run build — clean.
• npm test — 14/14 pass.
• Live-tested against a real AUTHORISED, tax-inclusive bank transaction: editing a line description now succeeds; the transaction's total, status, contact, and tax treatment are unchanged and the new description is persisted.