refactor: extract NATS.Server.Raft.Tests project

Move 43 Raft consensus test files (8 root-level + 35 in Raft/ subfolder)
from NATS.Server.Tests into a dedicated NATS.Server.Raft.Tests project.
Update namespaces, add InternalsVisibleTo, and fix timing/exception
handling issues in moved test files.

tests/NATS.Server.Raft.Tests/RaftElectionTests.cs

@@ -0,0 +1,81 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Raft.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; }
+    public RaftNode Leader { get; private set; }
+    public RaftNode LaggingFollower { get; private set; }
+
+    private RaftTestCluster(List<RaftNode> nodes)
+    {
+        Nodes = nodes;
+        Leader = nodes[0];
+        LaggingFollower = nodes[^1];
+    }
+
+    public static RaftTestCluster Create(int nodes)
+    {
+        var created = Enumerable.Range(1, nodes).Select(i => new RaftNode($"n{i}")).ToList();
+        foreach (var node in created)
+            node.ConfigureCluster(created);
+        return new RaftTestCluster(created);
+    }
+
+    public Task<RaftNode> ElectLeaderAsync()
+    {
+        var candidate = Nodes[0];
+        candidate.StartElection(Nodes.Count);
+
+        foreach (var voter in Nodes.Skip(1))
+            candidate.ReceiveVote(voter.GrantVote(candidate.Term));
+
+        Leader = candidate;
+        return Task.FromResult(candidate);
+    }
+
+    public Task WaitForAppliedAsync(long index)
+    {
+        if (Nodes.All(n => n.AppliedIndex >= index))
+            return Task.CompletedTask;
+
+        using var gate = new ManualResetEventSlim(false);
+        gate.Wait(TimeSpan.FromSeconds(2));
+
+        return Task.CompletedTask;
+    }
+
+    public async Task GenerateCommittedEntriesAsync(int count)
+    {
+        var leader = await ElectLeaderAsync();
+        for (int i = 0; i < count; i++)
+            _ = await leader.ProposeAsync($"cmd-{i}", default);
+    }
+
+    public Task RestartLaggingFollowerAsync()
+    {
+        LaggingFollower = Nodes[^1];
+        LaggingFollower.AppliedIndex = 0;
+        return Task.CompletedTask;
+    }
+
+    public async Task WaitForFollowerCatchupAsync()
+    {
+        var snapshot = await Leader.CreateSnapshotAsync(default);
+        await LaggingFollower.InstallSnapshotAsync(snapshot, default);
+    }
+}