Prevent duplicate delivery of a re-fetched already-delivered message

Under push-only delivery a delivered message position can be re-fetched (a
reopen removes it from the clearer's ignore-set before the asynchronous Clear
deletes it from the store) and routed to a fresh QueueManager on restart. That
instance reloaded the idempotency keys but discarded the delivered-positions, so
IdempotencyKeys.Add(sameKey, samePosition) returned true and the message was
delivered a second time.

Seed _fetchedPositions from the persisted DeliveredPositionsId on Initialize so
the existing ProcessMessages skip-check catches a re-fetched delivered position.
Only delivered/skipped positions are seeded, so a recorded-but-not-delivered
message still re-delivers correctly - no message loss.

Author: stidsborg

Core/Cleipnir.ResilientFunctions/Queuing/QueueManager.cs

@@ -86,6 +86,12 @@ private async Task Initialize()
 
             if (_effect.TryGet<List<long>>(DeliveredPositionsId, out var positions) && positions is { Count: > 0 })
             {
+                // Remember the positions a previous instance already delivered, so a message that is re-fetched
+                // before its Clear deletes it from the store is skipped here rather than delivered a second time.
+                lock (_lock)
+                    foreach (var position in positions)
+                        _fetchedPositions.Add(position);
+
                 await _messageClearer.Clear(positions);
                 positions.Clear();
                 _effect.FlushlessUpsert(DeliveredPositionsId, positions, alias: null);