fix: update bank transaction with minimal payload

[miachillgood]

Summary

• send a minimal bank transaction update payload instead of spreading the full fetched transaction back into the update request
• keep the existence check, but only include fields that the caller explicitly wants to change
• add a handler test that verifies line-item updates do not resend unrelated fields like status or bankTransactionID

Why

Issue #184 reports that update-bank-transaction fails whenever lineItems are included. The previous implementation reused the full bank transaction returned by Xero and sent that object back through updateBankTransaction. This can include read-only or response-only fields that are not needed for an update request and are more likely to break line item recategorisation flows.

Validation

• npm test
• npm run lint

Closes #184