Both the activation date and at least one line item identifier are mandatory; the request is
rejected with 400 Bad Request when either is missing.

The order itself must also be eligible: it must exist, must not already be activated, must be in
a status that permits activation, and must be flagged as primary for processing.

Note that isAsync is a request, not a guarantee. Setting it to false only
results in synchronous processing while the number of line items stays within the configured
threshold for synchronous activation. Above that threshold the work is queued regardless, and
beyond the configured batch size it is split into batches. Whenever the work is queued the
response carries a job identifier that can be polled through the job status endpoints.