test(batch8): resolve jetstream direct get up-to-time mapping

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamEngineTests.cs

@@ -1,6 +1,7 @@
 // Copyright 2025 The NATS Authors
 // Licensed under the Apache License, Version 2.0
 
+using System.Text;
 using Shouldly;
 using ZB.MOM.NatsNet.Server;
 
@@ -108,4 +109,61 @@ public sealed class JetStreamEngineTests
         err.ErrCode.ShouldBe(JsApiErrors.StreamInvalidConfig.ErrCode);
         err.Description.ShouldBe("stream configuration for republish destination forms a cycle");
     }
+
+    [Fact] // T:1751
+    public void JetStreamDirectGetUpToTime_ShouldSucceed()
+    {
+        const long unixEpochTicks = 621355968000000000L;
+        var baseTicks = DateTime.UnixEpoch.Ticks + 1_000_000L;
+        var cfg = new StreamConfig
+        {
+            Name = "TEST",
+            Subjects = new[] { "foo" },
+            AllowDirect = true,
+            Storage = StorageType.MemoryStorage,
+        };
+
+        var ms = JetStreamMemStore.NewMemStore(cfg);
+        var timestamps = new List<DateTime>(10);
+
+        for (var i = 0; i < 10; i++)
+        {
+            var ticks = baseTicks + i;
+            var ts = (ticks - unixEpochTicks) * 100L;
+            ms.StoreRawMsg("foo", null, Encoding.UTF8.GetBytes($"message {i + 1}"), (ulong)(i + 1), ts, 0, true);
+            timestamps.Add(new DateTime(ticks, DateTimeKind.Utc));
+        }
+
+        static string[] CheckResponses(IStreamStore store, DateTime upToTime)
+        {
+            var state = store.State();
+            var upToSeq = store.GetSeqFromTime(upToTime);
+            if (upToSeq <= state.FirstSeq)
+                return Array.Empty<string>();
+
+            upToSeq--;
+            if (upToSeq == 0)
+                upToSeq = state.LastSeq;
+
+            var (seqs, err) = store.MultiLastSeqs(new[] { "foo" }, upToSeq, 1024);
+            err.ShouldBeNull();
+            if (seqs is null || seqs.Length == 0)
+                return Array.Empty<string>();
+
+            var messages = new List<string>(seqs.Length);
+            foreach (var seq in seqs)
+            {
+                var sm = store.LoadMsg(seq, null);
+                sm.ShouldNotBeNull();
+                messages.Add(Encoding.UTF8.GetString(sm!.Msg));
+            }
+
+            return messages.ToArray();
+        }
+
+        CheckResponses(ms, DateTime.UnixEpoch).ShouldBe(Array.Empty<string>());
+        CheckResponses(ms, new DateTime(2100, 1, 1, 0, 0, 0, DateTimeKind.Utc)).ShouldBe(new[] { "message 10" });
+        CheckResponses(ms, timestamps[0]).ShouldBe(Array.Empty<string>());
+        CheckResponses(ms, timestamps[4]).ShouldBe(new[] { "message 4" });
+    }
 }