feat: define jetstream storage interfaces

tests/NATS.Server.Tests/StreamStoreContractTests.cs

@@ -0,0 +1,31 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests;
+
+public class StreamStoreContractTests
+{
+    [Fact]
+    public async Task Append_increments_sequence_and_updates_state()
+    {
+        var store = new FakeStreamStore();
+        var seq = await store.AppendAsync("foo", "bar"u8.ToArray(), default);
+
+        seq.ShouldBe((ulong)1);
+        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)1);
+    }
+
+    private sealed class FakeStreamStore : IStreamStore
+    {
+        private ulong _last;
+
+        public ValueTask<ulong> AppendAsync(string subject, ReadOnlyMemory<byte> payload, CancellationToken ct)
+        {
+            _last++;
+            return ValueTask.FromResult(_last);
+        }
+
+        public ValueTask<StreamState> GetStateAsync(CancellationToken ct)
+            => ValueTask.FromResult(new StreamState { Messages = _last });
+    }
+}