The mapped Subscription carried status and dates but not its items, so
which plan and which billing cycle a subscription is actually on could not
be read at all. A caller deciding whether a requested plan change is real
had nothing to compare against, and would bill a proration for an update
that changed nothing.

Added as a defaulted constructor argument so existing construction sites
are untouched; fromSdk always populates it.

Split from 62fcfc24c5ea257ad5bf2f01ec041b8bd33546a2