refactor: extract NATS.Server.JetStream.Tests project

Move 225 JetStream-related test files from NATS.Server.Tests into a
dedicated NATS.Server.JetStream.Tests project. This includes root-level
JetStream*.cs files, storage test files (FileStore, MemStore,
StreamStoreContract), and the full JetStream/ subfolder tree (Api,
Cluster, Consumers, MirrorSource, Snapshots, Storage, Streams).

Updated all namespaces, added InternalsVisibleTo, registered in the
solution file, and added the JETSTREAM_INTEGRATION_MATRIX define.

tests/NATS.Server.JetStream.Tests/JetStreamStreamReplicaGroupTests.cs

@@ -0,0 +1,71 @@
+using System.Text;
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Publish;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamReplicaGroupTests
+{
+    [Fact]
+    public async Task Leader_stepdown_preserves_stream_write_availability_after_new_election()
+    {
+        await using var fixture = await JetStreamReplicaFixture.StartAsync(nodes: 3);
+        await fixture.CreateStreamAsync("ORDERS", replicas: 3);
+
+        await fixture.StepDownStreamLeaderAsync("ORDERS");
+        var ack = await fixture.PublishAndGetAckAsync("orders.created", "1");
+
+        ack.Stream.ShouldBe("ORDERS");
+        ack.Seq.ShouldBeGreaterThan((ulong)0);
+    }
+}
+
+internal sealed class JetStreamReplicaFixture : IAsyncDisposable
+{
+    private readonly StreamManager _streamManager;
+    private readonly JetStreamPublisher _publisher;
+
+    private JetStreamReplicaFixture(StreamManager streamManager)
+    {
+        _streamManager = streamManager;
+        _publisher = new JetStreamPublisher(_streamManager);
+    }
+
+    public static Task<JetStreamReplicaFixture> StartAsync(int nodes)
+    {
+        _ = nodes;
+        var streamManager = new StreamManager();
+        return Task.FromResult(new JetStreamReplicaFixture(streamManager));
+    }
+
+    public Task CreateStreamAsync(string name, int replicas)
+    {
+        var response = _streamManager.CreateOrUpdate(new StreamConfig
+        {
+            Name = name,
+            Subjects = ["orders.*"],
+            Replicas = replicas,
+        });
+
+        if (response.Error is not null)
+            throw new InvalidOperationException(response.Error.Description);
+
+        return Task.CompletedTask;
+    }
+
+    public Task StepDownStreamLeaderAsync(string stream)
+    {
+        return _streamManager.StepDownStreamLeaderAsync(stream, default);
+    }
+
+    public Task<PubAck> PublishAndGetAckAsync(string subject, string payload)
+    {
+        if (_publisher.TryCapture(subject, Encoding.UTF8.GetBytes(payload), null, out var ack))
+            return Task.FromResult(ack);
+
+        throw new InvalidOperationException("Publish did not match a stream.");
+    }
+
+    public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+}