Set completed to true to finish an external task or false to reopen it.
evidence must be a JSON object and externalOperationId is required.
Operation IDs are durable and workspace-scoped. Retrying the same operation ID
with the same instance, desired state, and evidence returns the task’s current
state without publishing another event. Reusing it with a different instance,
state, or evidence returns ALREADY_EXISTS (HTTP 409).
Only instances whose snapshotted type is external can be updated.