feat: implement raft election and term state

tests/NATS.Server.Tests/RaftElectionTests.cs

@@ -0,0 +1,43 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Tests;
+
+public class RaftElectionTests
+{
+    [Fact]
+    public async Task Candidate_becomes_leader_after_majority_votes()
+    {
+        var cluster = RaftTestCluster.Create(3);
+        var leader = await cluster.ElectLeaderAsync();
+
+        leader.Role.ShouldBe(RaftRole.Leader);
+        leader.Term.ShouldBe(1);
+    }
+}
+
+internal sealed class RaftTestCluster
+{
+    public List<RaftNode> Nodes { get; }
+
+    private RaftTestCluster(List<RaftNode> nodes)
+    {
+        Nodes = nodes;
+    }
+
+    public static RaftTestCluster Create(int nodes)
+    {
+        var created = Enumerable.Range(1, nodes).Select(i => new RaftNode($"n{i}")).ToList();
+        return new RaftTestCluster(created);
+    }
+
+    public Task<RaftNode> ElectLeaderAsync()
+    {
+        var candidate = Nodes[0];
+        candidate.StartElection(Nodes.Count);
+
+        foreach (var voter in Nodes.Skip(1))
+            candidate.ReceiveVote(voter.GrantVote(candidate.Term));
+
+        return Task.FromResult(candidate);
+    }
+}