Returns the current state of a batch import and the per-file outcomes.

The response includes one results[] entry per uploaded file, in the original upload order. Each result carries an explicit index (its zero-based position in the upload, the reliable correlation key), its status (unfinished, done, error, done_with_error, etc.), the resulting invoice_id when applicable, and the importer error message when the import failed.

While the batch is still processing, items not yet picked up by the job report status: unfinished. Once the batch reaches done, all items have a terminal status.