feat(batch14): complete group1 write path and wave1 tests

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/ImplBacklog/ConcurrencyTests1.Impltests.cs

@@ -1,4 +1,5 @@
 using System.Collections.Concurrent;
+using System.Diagnostics;
 using Shouldly;
 using ZB.MOM.NatsNet.Server;
 
@@ -134,6 +135,47 @@ public sealed partial class ConcurrencyTests1
         }
     }
 
+    [Fact] // T:2441
+    public void NoRaceJetStreamFileStoreLargeKVAccessTiming_ShouldSucceed()
+    {
+        var value = Enumerable.Repeat((byte)'Z', 256).ToArray();
+        const int keyCount = 5_000;
+
+        WithStore((fs, _) =>
+        {
+            for (var i = 1; i <= keyCount; i++)
+            {
+                fs.StoreMsg($"KV.STREAM_NAME.{i}", null, value, 0).Seq.ShouldBeGreaterThan(0UL);
+            }
+
+            var sw = Stopwatch.StartNew();
+            var last = fs.LoadLastMsg($"KV.STREAM_NAME.{keyCount}", null);
+            sw.Stop();
+            var lastLookup = sw.Elapsed;
+
+            last.ShouldNotBeNull();
+            last!.Msg.ShouldBe(value);
+
+            sw.Restart();
+            var first = fs.LoadLastMsg("KV.STREAM_NAME.1", null);
+            sw.Stop();
+            var firstLookup = sw.Elapsed;
+
+            first.ShouldNotBeNull();
+            first!.Msg.ShouldBe(value);
+
+            // Keep generous bounds to avoid machine-specific flakiness while still
+            // asserting access stays fast under a large key set.
+            lastLookup.ShouldBeLessThan(TimeSpan.FromMilliseconds(250));
+            firstLookup.ShouldBeLessThan(TimeSpan.FromMilliseconds(350));
+
+            var firstState = fs.FilteredState(1, "KV.STREAM_NAME.1");
+            var lastState = fs.FilteredState(1, $"KV.STREAM_NAME.{keyCount}");
+            firstState.First.ShouldBeGreaterThan(0UL);
+            lastState.First.ShouldBeGreaterThan(0UL);
+        }, DefaultStreamConfig());
+    }
+
     private static void WithStore(Action<JetStreamFileStore, string> action, StreamConfig? cfg = null)
     {
         var root = NewRoot();