fix: Issue with Fixed Term subscriptions created with 0.22.x - #2309

Fixes #1909

Root cause

Legacy FIXEDTERM subscriptions have no EXPIRED event, so billing never stops

Changes

• In DefaultSubscriptionBase.getSubscriptionBillingEvents, when no CANCEL/EXPIRED transition exists and the last Plan/PlanPhase transition lands on the plan's final FIXEDTERM phase, derive an EXPIRED billing event at phaseStart + phase duration (dropping catalog-version change candidates at/after that date), which stops billing exactly like the persisted EXPIRED event does for subscriptions created with 0.24.x. Added fast regression tests.