Add a specialty to a facility

Adds a new specialty association to an existing facility. Use this to associate a single specialty with a facility; to apply multiple specialty changes in one call, use PATCH /facilities/{id}/specialties instead. Cascade to each additional level is not a separate flag — it is triggered purely by which optional ID fields are present in the request body: certifyLocationId alone cascades to the facility-location level; certifyGroupId cascades to the group level; certifyLocationId together with locationSpecialtyEffectiveDate cascades to the group-location level; certifyNetworkId cascades to the group-network level. Cascade levels are attempted independently — a failure on one level does not roll back or block the others; any level that fails is reported in the notCreated array of the response rather than failing the whole request.