feat: implement raft log replication and apply

tests/NATS.Server.Tests/RaftElectionTests.cs

@@ -27,6 +27,8 @@ internal sealed class RaftTestCluster
     public static RaftTestCluster Create(int nodes)
     {
         var created = Enumerable.Range(1, nodes).Select(i => new RaftNode($"n{i}")).ToList();
+        foreach (var node in created)
+            node.ConfigureCluster(created);
         return new RaftTestCluster(created);
     }
 
@@ -40,4 +42,16 @@ internal sealed class RaftTestCluster
 
         return Task.FromResult(candidate);
     }
+
+    public async Task WaitForAppliedAsync(long index)
+    {
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(2));
+        while (!timeout.IsCancellationRequested)
+        {
+            if (Nodes.All(n => n.AppliedIndex >= index))
+                return;
+
+            await Task.Delay(20, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);
+        }
+    }
 }

tests/NATS.Server.Tests/RaftReplicationTests.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Tests;
+
+public class RaftReplicationTests
+{
+    [Fact]
+    public async Task Leader_replicates_entry_to_quorum_and_applies()
+    {
+        var cluster = RaftTestCluster.Create(3);
+        var leader = await cluster.ElectLeaderAsync();
+
+        var idx = await leader.ProposeAsync("create-stream", default);
+        idx.ShouldBeGreaterThan(0);
+
+        await cluster.WaitForAppliedAsync(idx);
+        cluster.Nodes.All(n => n.AppliedIndex >= idx).ShouldBeTrue();
+    }
+}