Sync: take the server having no such item as a delete done, not a rejection nobody can clear

src/DodoSSH.Client.Sync/SyncEngine.cs

@@ -539,6 +539,17 @@ public sealed class SyncEngine
         return resealed;
     }
 
+    /// <summary>Whether a refused operation is a delete of an item the server never had.</summary>
+    /// <remarks>
+    /// A delete queued over a create that never reached the server carries no expected version, and the
+    /// server's only reason to refuse one of those is that it has no such item — which is the state the delete
+    /// asked for, so it is done. It used to be parked as a rejection: a "conflict" about an item nobody else
+    /// had ever seen, which the user could neither resolve nor dismiss. A create that did land is no exception
+    /// to this — the server has the item then, and applies the delete.
+    /// </remarks>
+    private static bool DeletesSomethingNeverCreated(PendingOperation operation) =>
+        operation is { Operation: SyncOperation.Delete, ExpectedVersion: null };
+
     /// <returns>Whether this answer warrants another push round.</returns>
     private async Task<bool> HandleAsync(
         Guid vaultId,
@@ -581,6 +592,10 @@ public sealed class SyncEngine
                     cancellationToken).ConfigureAwait(false);
                 return false;
 
+            case SyncOperationStatus.Invalid when DeletesSomethingNeverCreated(operation):
+                await outbox.CompleteAsync(operation, cancellationToken).ConfigureAwait(false);
+                return false;
+
             case SyncOperationStatus.Invalid:
                 await RejectAsync(
                     vaultId,