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/RaftTransportPersistenceTests.cs

@@ -0,0 +1,90 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Raft.Tests;
+
+public class RaftTransportPersistenceTests
+{
+    [Fact]
+    public async Task Raft_node_recovers_log_and_term_after_restart()
+    {
+        await using var fx = await RaftFixture.StartPersistentClusterAsync();
+        var idx = await fx.Leader.ProposeAsync("cmd", default);
+        await fx.RestartNodeAsync("n2");
+        (await fx.ReadNodeAppliedIndexAsync("n2")).ShouldBeGreaterThanOrEqualTo(idx);
+    }
+}
+
+internal sealed class RaftFixture : IAsyncDisposable
+{
+    private readonly string _root;
+    private readonly InMemoryRaftTransport _transport;
+    private readonly Dictionary<string, RaftNode> _nodes;
+
+    private RaftFixture(string root, InMemoryRaftTransport transport, Dictionary<string, RaftNode> nodes)
+    {
+        _root = root;
+        _transport = transport;
+        _nodes = nodes;
+    }
+
+    public RaftNode Leader => _nodes["n1"];
+
+    public static Task<RaftFixture> StartPersistentClusterAsync()
+    {
+        var root = Path.Combine(Path.GetTempPath(), $"nats-raft-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(root);
+
+        var transport = new InMemoryRaftTransport();
+        var nodes = new Dictionary<string, RaftNode>(StringComparer.Ordinal);
+        foreach (var id in new[] { "n1", "n2", "n3" })
+        {
+            var node = new RaftNode(id, transport, Path.Combine(root, id));
+            transport.Register(node);
+            nodes[id] = node;
+        }
+
+        var all = nodes.Values.ToArray();
+        foreach (var node in all)
+            node.ConfigureCluster(all);
+
+        var leader = nodes["n1"];
+        leader.StartElection(all.Length);
+        leader.ReceiveVote(nodes["n2"].GrantVote(leader.Term), all.Length);
+        leader.ReceiveVote(nodes["n3"].GrantVote(leader.Term), all.Length);
+
+        return Task.FromResult(new RaftFixture(root, transport, nodes));
+    }
+
+    public async Task RestartNodeAsync(string id)
+    {
+        var nodePath = Path.Combine(_root, id);
+        var replacement = new RaftNode(id, _transport, nodePath);
+        await replacement.LoadPersistedStateAsync(default);
+        _transport.Register(replacement);
+        _nodes[id] = replacement;
+
+        var all = _nodes.Values.ToArray();
+        foreach (var node in all)
+            node.ConfigureCluster(all);
+    }
+
+    public Task<long> ReadNodeAppliedIndexAsync(string id)
+    {
+        return Task.FromResult(_nodes[id].AppliedIndex);
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        try
+        {
+            if (Directory.Exists(_root))
+                Directory.Delete(_root, recursive: true);
+        }
+        catch (IOException ex)
+        {
+            System.Diagnostics.Debug.WriteLine($"Failed to clean up temp directory {_root}: {ex.Message}");
+        }
+
+        return ValueTask.CompletedTask;
+    }
+}