feat: implement raft log replication and apply

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();
+    }
+}