fix: stop accumulating WhenAny continuations on the Conditions aggregate

[beekld]

The FDv2 data system's Conditions::GetFuture() previously returned the same shared future to every iteration of RunSynchronizerNext, so each iteration's WhenAny(cond_future, next_future).Then(...) appended a callback to the shared future's continuations_ vector. On a healthy primary streaming changesets without ever firing fallback/recovery, those continuations accumulated for the synchronizer's lifetime.

• Conditions::GetFuture(token) now returns a fresh Future per call.
  • The caller cancels its CancellationSource after the WhenAny resolves.
  • The cancellation callback erases the per-call Promise from a pending list inside Conditions.
• A single permanent listener on the underlying aggregate drains the pending list and resolves each Promise when any condition fires.

Analogous to launchdarkly/java-core#163.