Fix race condition in EventConsumerProcessor.CompleteAsync causing flaky test (#1306)

* Fix race condition in EventConsumerProcessor.CompleteAsync

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Co-authored-by: SebastianStehle <1236435+SebastianStehle@users.noreply.github.com>

* Replace empty using block with idiomatic using-var discard

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Co-authored-by: SebastianStehle <1236435+SebastianStehle@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: SebastianStehle <1236435+SebastianStehle@users.noreply.github.com>

backend/src/Squidex.Infrastructure/EventSourcing/Consume/EventConsumerProcessor.cs

@@ -72,6 +72,9 @@ public class EventConsumerProcessor : IEventSubscriber<ParsedEvents>
                 log.LogCritical(ex, "Failed to complete consumer.");
             }
         }
+
+        // Acquire the lock to ensure any in-flight UpdateAsync has fully completed before returning.
+        using var _ = await asyncLock.EnterAsync();
     }
 
     public virtual ValueTask OnNextAsync(IEventSubscription subscription, ParsedEvents @event)