Set practitioner network enrollment start date

Sets the start date of a practitioner's enrollment in a network group, identified by networkId and tenantGroupPractitionerId in the request body. effectiveDate must not be later than the enrollment's existing termination date, or a 400 is returned. This operation is idempotent (upsert semantics). Use this endpoint to set the enrollment start date only; to update specialty-level dates use PUT /networks/{networkId}/groups/{groupId}/practitioners/{practitionerId}/specialties/{specialtyId}/dates instead. Requires the UPDATE_NETWORK permission.