Prevent canceled @stream tasks from breaking subsequent @stream query

[atykhyy]

This fix largely prevents canceled @stream tasks from breaking subsequent @stream queries.

When a query containing deferred @stream is canceled (e.g. due to http connection closing), DeferResultStream.GetAsyncEnumerator() disposes the work state owner, invoking DeferredWorkState.Reset(), before the tasks waiting for items from async enumerators being @stream'ed are canceled. When these tasks are canceled, they add IQueryResults with Errors set to OperationCanceledException to the freshly reset DeferredWorkState. The next query which rents this DeferredWorkState picks up these stale results and quits in DeferredWorkState.TryDequeueResultsAsync() after sending its initial result, so that the client invoking the query receives an empty result with hasNext:false and no @stream items.

ETA: this change doesn't fix the problem completely, but it does reduce the window of the underlying race condition very substantially. Instead of every other subscription being truncated as described above, I now observe truncation rarely and sporadically.

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