New `import_source` field on Supplier Invoices endpoints

The response of the supplier invoices endpoints now includes an import_source object. For invoices imported via a file connector (Dropbox or Google Drive), it returns the connector type and the file path in the connector's storage. For invoices created by other means, or when path information is unavailable, the field is null.

Impacted endpoints:

• List supplier invoices
• Retrieve a supplier invoice
• Import a supplier invoice with a file attached
• Update a supplier invoice
• Validate the accounting of a supplier invoice

This is a non-breaking change. The new field is additional and does not affect the existing response structures.