NotifyManager class Managers

Batches cache notifications so one cascade of cache writes produces one round of listener calls.

Each client owns one, as QueryClient.notifyManager; most apps never touch it. Two differences from TanStack Query:

• The default scheduler is scheduleMicrotask, not a zero-delay timer. It is faster, it is deterministic, and fake_async controls it.
• This is an ordinary object rather than a module-level singleton, so a QueryClient can own one and tests are hermetic. A client constructed without one creates its own; apps that want a single shared queue pass NotifyManager.shared to every client.

A batch holds callbacks submitted through schedule or batchCalls until the outermost batch ends, then delivers them through the scheduler. Direct observer subscriptions, cache listeners and onQueryUpdate remain synchronous per dispatch. Wrap a subscription with batchCalls when its delivery should be deferred. A throwing callback in a queued batch is reported to the zone and does not discard later callbacks in that batch.

// Several writes; callbacks queued through `schedule` or `batchCalls`
// meanwhile are delivered in one round after the batch ends:
client.notifyManager.batch(() {
  client.setQueryData<int>(QueryKey(['a']), 1);
  client.setQueryData<int>(QueryKey(['b']), 2);
});

// A cache subscription whose calls are deferred and batched:
client.queryCache.subscribe(
  client.notifyManager.batchCalls((QueryCacheEvent event) => log(event)),
);