Resync the Reference field for an invoice that was already synced to Xero.

Use this when the initial sync didn't include a Reference (clearing_number/payment_ref), but now has that data available in the line items.

The reference is built using the same logic as the initial sync:

• If both clearing_number and payment_ref exist: "{clearing_number}_{payment_ref}"
• If only clearing_number exists: "{clearing_number}"
• If only payment_ref exists: "{payment_ref}"