Invoice Lines endpoints - Adding `invoice_number` in the response

We're now returning the invoice_number attribute in the response of the Ledger Entries endpoints, offering more context about the document linked to a ledger entry.

Impacted endpoints:

• List Ledger Entries
• Retrieve a Ledger entry
• Create a ledger entry
• Update a ledger entry

This is a non-breaking change – existing integrations will continue to work without modification. The new field is additional and does not affect the existing response structures.