Second step of an import, to call once the ZIP archive has been uploaded to the uploadUrl returned by initiateDataImport. Marks the record as UPLOADED and dispatches it for processing (queued per creating user, or launched as a processing task); progress is then followed with getDataImportRecord.

Idempotent: calling it again for a record that is no longer INITIATED does not start a second import; it returns the current record (and re-queues an UPLOADED record that has not started yet).

Authorization: any authenticated user; the record must belong to the caller's tenant.

Errors: 400 if the record does not exist in the caller's tenant; 500 if the provider has no processing task configured.