Returns the participants of each regular payroll run: the employees whose payroll results
compensations/payroll_results bulk_create accepts. Use it before an import to send results
only for these employees.

Off-cycle runs (any payment_type other than regular, see compensations/payroll_runs) are
rejected. They have no fixed population: most of the company's employees are likely to be
valid for one, so a list from this endpoint would not provide much value when trying to
know who an off-cycle payment is for. Knowing which employees to sync is a task the
integration developer should be responsible for: post them directly to bulk_create, whose
employee checks always act as a guardrail and reject any employee who isn't valid for the
run.

The list is not paginated: every participant is returned in one response. It only
includes employees the caller is allowed to see.