Internal Cloud Tasks callback — create one workflow in bulk job

Internal callback endpoint invoked by Google Cloud Tasks as part of the v2 bulk-create flow. Creates a single credentialing workflow for one practitioner and updates the corresponding bulk request item status on completion. This endpoint is not intended for direct client use — authenticate with the X-Credentialing-Api-Key header; JWT authentication is not used. Required body fields: certifyPractitionerId, tenantPractitionerId, credentialingWorkflowBulkRequestId, credentialingWorkflowBulkRequestItemId. Optional: data object containing the workflow creation payload.