Returns the current state of a subscription.

Subscriptions are versioned. Each change to a subscription — quantity, billing term,
price, or product — creates a new subscription record that belongs to the same lineage
as the original. You may supply the ID of any record in that lineage; this endpoint
always resolves to the most recent record in it.

Because of this, the id in the response is the lineage's original subscription ID,
which may differ from the subscriptionId you supplied. This is expected and does not
indicate an error. The original subscription ID is stable for the life of the
subscription, so prefer it when storing a reference.

To see every record in the lineage, including the one matching the ID you supplied, pass
the id from this response to GET /subscriptions/{subscriptionId}/history.

Returns 404 if no subscription matches the supplied subscriptionId.