Registers customers in batch mode (maximum 1000 items per request) and returns
per-customer registration results.

The response includes one result for each processed customer and can be correlated by extId
or email.

For each successful result, extId is generated if it was not provided in the request.
Store this value on the client side as the partner customer identifier for future customer-related
partner endpoints.

Failed results include error details.

This endpoint always returns 200 OK for business-level results, including partial success
or full business-level failure.

To register a single customer, send a request array containing one item.