Implement in-process multi-dataset sync isolation across core, network, persistence, and tests

tests/ZB.MOM.WW.CDBBC.E2E.Benchmark.Tests/BenchmarkPeerNode.cs

@@ -113,12 +113,7 @@ internal sealed class BenchmarkPeerNode : IAsyncDisposable
 
     public async Task UpsertUserAsync(User user)
     {
-        User? existing = Context.Users.Find(u => u.Id == user.Id).FirstOrDefault();
-        if (existing == null)
-            await Context.Users.InsertAsync(user);
-        else
-            await Context.Users.UpdateAsync(user);
-
+        await Context.Users.UpdateAsync(user);
         await Context.SaveChangesAsync();
     }
 
@@ -127,6 +122,11 @@ internal sealed class BenchmarkPeerNode : IAsyncDisposable
         return Context.Users.Find(u => u.Id == userId).Any();
     }
 
+    public int CountUsersWithPrefix(string prefix)
+    {
+        return Context.Users.FindAll().Count(u => u.Id.StartsWith(prefix, StringComparison.Ordinal));
+    }
+
     public async ValueTask DisposeAsync()
     {
         try