Persist deduplicated message positions so they are always cleared (#176)

When ProcessMessages drops a message as a duplicate (by idempotency key),
stage its position into _deliveredPositions and persist it to the
DeliveredPositionsId effect immediately, rather than relying on an
immediate per-position store delete.

Previously the dedup branch added the position to the in-memory set only
when piggybacking on a real delivery's effect upsert. If no delivery
followed (all-duplicate batch, no matching subscription, or the flow
suspended/completed first) the position was never written to the effect,
so AfterFlush never cleared it from the store and a crash could not
replay it. Persisting at stage time routes dedup cleanup through the same
batched, crash-safe drain path as real deliveries. The write is taken
under _lock to match the DeliverMessages access.

Author: stidsborg

Core/Cleipnir.ResilientFunctions/Queuing/QueueManager.cs

@@ -227,7 +227,11 @@ private async Task ProcessMessages(IReadOnlyList<StoredMessage> messages)
 
                 if (idempotencyKey != null && !_idempotencyKeys.Add(idempotencyKey, position))
                 {
-                    await _messageClearer.Clear([position]);
+                    lock (_lock)
+                    {
+                        _deliveredPositions.Add(position);
+                        _effect.FlushlessUpsert(DeliveredPositionsId, _deliveredPositions.ToList(), alias: null);
+                    }
                     continue;
                 }