Retrieve the overall progress and row counters for a bulk mailbox import.

• Poll this endpoint to track an import until its status reaches completed.
• The import is complete when processed_rows equals queued_rows. If every row was rejected at upload, the import is completed straight away.
• In rare cases an import can stay processing after its rows have stopped moving. Short pauses are normal, especially when several imports run at the same time. If processed_rows hasn't changed for 1 hour while the import is still processing, treat it as stuck: check which rows are still pending with List Import Rows (status=processing and status=queued), then contact the Mailivery Support team with the import ID.
• Row counters satisfy: processed_rows = created_rows + failed_rows.
• Import lookup is scoped strictly to the authenticated caller's account.