feat: complete remaining jetstream parity implementation plan

tests/NATS.Server.Tests/JetStreamStoreIndexTests.cs

@@ -0,0 +1,18 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests;
+
+public class JetStreamStoreIndexTests
+{
+    [Fact]
+    public async Task Store_can_get_last_message_by_subject()
+    {
+        var store = new MemStore();
+        await store.AppendAsync("orders.created", "1"u8.ToArray(), default);
+        await store.AppendAsync("orders.updated", "2"u8.ToArray(), default);
+        await store.AppendAsync("orders.created", "3"u8.ToArray(), default);
+
+        var last = await store.LoadLastBySubjectAsync("orders.created", default);
+        last!.Payload.Span.SequenceEqual("3"u8).ShouldBeTrue();
+    }
+}