Activates an order, making it live and effective in the system. This typically means the order
starts generating revenue, billing cycles begin, or services/products become available to the customer.
The activation date determines when the order takes effect and influences billing calculations.

The activation date is mandatory; the request is rejected with 400 Bad Request when it is missing.
The order must also 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 order's line item count 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.