fix: re-enable leader check in API router and fix stepdown simulation

Re-enabled the leader check in JetStreamApiRouter.Route() that was
commented out during B8 agent work. Added BecomeLeader() method to
JetStreamMetaGroup for single-process test fixtures to simulate
winning the post-stepdown election. MetaControllerFixture now
auto-calls BecomeLeader() after a successful meta leader stepdown.

src/NATS.Server/JetStream/Api/JetStreamApiRouter.cs

@@ -102,15 +102,11 @@ public sealed class JetStreamApiRouter
 
     public JetStreamApiResponse Route(string subject, ReadOnlySpan<byte> payload)
     {
-        // TODO: Re-enable leader check once ForwardToLeader is implemented with actual
-        // request forwarding to the leader node. Currently ForwardToLeader is a stub that
-        // returns a not-leader error, which breaks single-node simulation tests where
-        // the meta group's selfIndex doesn't track the rotating leader.
         // Go reference: jetstream_api.go:200-300 — leader check + forwarding.
-        // if (_metaGroup is not null && IsLeaderRequired(subject) && !_metaGroup.IsLeader())
+        if (_metaGroup is not null && IsLeaderRequired(subject) && !_metaGroup.IsLeader())
-        // {
+        {
-        //     return ForwardToLeader(subject, payload, _metaGroup.Leader);
+            return ForwardToLeader(subject, payload, _metaGroup.Leader);
-        // }
+        }
 
         if (subject.Equals(JetStreamApiSubjects.Info, StringComparison.Ordinal))
             return AccountApiHandlers.HandleInfo(_streamManager, _consumerManager);

src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs

@@ -12,7 +12,7 @@ namespace NATS.Server.JetStream.Cluster;
 public sealed class JetStreamMetaGroup
 {
     private readonly int _nodes;
-    private readonly int _selfIndex;
+    private int _selfIndex;
 
     // Backward-compatible stream name set used by existing GetState().Streams.
     private readonly ConcurrentDictionary<string, byte> _streams = new(StringComparer.Ordinal);
@@ -50,6 +50,13 @@ public sealed class JetStreamMetaGroup
     /// </summary>
     public bool IsLeader() => _leaderIndex == _selfIndex;
 
+    /// <summary>
+    /// Simulates this node winning the leader election after a stepdown.
+    /// Used in single-process test fixtures where only one "node" exists.
+    /// Go reference: jetstream_cluster.go — after stepdown, a new leader is elected.
+    /// </summary>
+    public void BecomeLeader() => _selfIndex = _leaderIndex;
+
     /// <summary>
     /// Returns the leader identifier string, e.g. "meta-1".
     /// Used to populate the leader_hint field in not-leader error responses.

tests/NATS.Server.Tests/JetStream/Cluster/JetStreamMetaControllerTests.cs

@@ -625,7 +625,17 @@ internal sealed class MetaControllerFixture : IAsyncDisposable
     public MetaGroupState GetMetaState() => _metaGroup.GetState();
 
     public Task<JetStreamApiResponse> RequestAsync(string subject, string payload)
-        => Task.FromResult(_router.Route(subject, Encoding.UTF8.GetBytes(payload)));
+    {
+        var response = _router.Route(subject, Encoding.UTF8.GetBytes(payload));
+
+        // In a real cluster, after stepdown a new leader is elected.
+        // Simulate this node becoming the new leader so subsequent mutating
+        // operations through the router succeed.
+        if (subject.Equals(JetStreamApiSubjects.MetaLeaderStepdown, StringComparison.Ordinal) && response.Success)
+            _metaGroup.BecomeLeader();
+
+        return Task.FromResult(response);
+    }
 
     public ValueTask DisposeAsync() => ValueTask.CompletedTask;
 }