Extract Vector Set cleanup work channel and work set - #2020

Pulls the ad-hoc Channel and ConcurrentDictionary plumbing in the Vector Set cleanup pipeline behind two small types, and uses them in VectorManager. Behaviour is unchanged.

VectorSetCleanupWorkChannel<T>

Replaces all three queues. They differ only in how the consumer reads:

The drop queue's items are pure wake-ups - the work lives in requestedDrops, and one pass services all of it - so the backlog genuinely collapses and

// Drain all wake up signals
while (requestDropTaskChannel.Reader.TryRead(out _)) { }

gets a home as requestDropTaskChannel.DrainPending().

The cleanup-request queue also reads everything available, but for the opposite reason: each item is distinct work (a context to mark, and a TaskCompletionSource a caller is waiting on), and draining it in one go lets the pass mark every context and then persist UpdateContextMetadata once rather than once per item.

The last queue acts on the item itself, so it takes one per iteration.

Two of the queues carry no payload, because the work they refer to lives elsewhere - in requestedDrops for the drop queue, and in the marked contexts for the cleanup queue. Both DrainPending() and the payload-free TryPublish() are extension methods on VectorSetCleanupWorkChannel<object> rather than members, so neither can be called on requestCleanupTaskChannel: publishing a default tuple there would queue a null TaskCompletionSource, and discarding its backlog would drop contexts that need marking and strand every waiter on it.

The type also folds the three-step shutdown

requestDropTaskChannel.Writer.Complete();
AsyncUtils.BlockingWait(requestDropTaskChannel.Reader.Completion);
AsyncUtils.BlockingWait(requestDropTask);

into requestDropTaskChannel.CompleteAndWaitForConsumerTask(requestDropTask), and does not expose the underlying ChannelReader/ChannelWriter.

VectorSetCleanupWorkSet<TValue>

Wraps requestedDrops, moving the NET9_0_OR_GREATER alternate-lookup branching and the poll-until-absent loop out of VectorManager. DropRequested and WaitForDiskANNIndexDrop become one-liners over Contains / WaitForCompletion.

Tests

Unit tests over both types.