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.

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Note

Medium Risk
Touches FDv2 orchestration and async cancellation ordering (locks around CancellationCallback); behavior change is localized but long-running streaming paths depend on correct cleanup.

Overview
Fixes unbounded growth of WhenAny continuation callbacks during long-lived primary streaming, where each RunSynchronizerNext loop reused one shared Conditions future.

Conditions::GetFuture now takes a CancellationToken and returns a new future per call. A single internal aggregate listener still watches the first condition to fire; when it resolves, it fans out to all pending per-call promises. Close also drains any still-pending waiters with kCancelled.

Callers (notably FDv2DataSystem::RunSynchronizerNext) create a per-iteration CancellationSource, pass its token into GetFuture, and cancel after WhenAny completes so the pending entry and its promise/continuations can be dropped. Token cancellation removes the waiter from the pending list if the race finishes early.

Tests updated to pass a CancellationToken into GetFuture.

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