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/JetStreamMetaGroupTests.cs

@@ -0,0 +1,65 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Api;
+using NATS.Server.JetStream.Cluster;
+using NATS.Server.JetStream.Models;
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamMetaGroupTests
+{
+    [Fact]
+    public async Task Stream_create_requires_meta_group_commit()
+    {
+        await using var fixture = await MetaGroupTestFixture.StartAsync(nodes: 3);
+
+        var result = await fixture.CreateStreamAsync("ORDERS", replicas: 3);
+        result.Error.ShouldBeNull();
+
+        var meta = await fixture.GetMetaStateAsync();
+        meta.Streams.ShouldContain("ORDERS");
+    }
+}
+
+internal sealed class MetaGroupTestFixture : IAsyncDisposable
+{
+    private readonly JetStreamMetaGroup _metaGroup;
+    private readonly StreamManager _streamManager;
+    private readonly ConsumerManager _consumerManager;
+    private readonly JetStreamApiRouter _router;
+
+    private MetaGroupTestFixture(JetStreamMetaGroup metaGroup, StreamManager streamManager, ConsumerManager consumerManager, JetStreamApiRouter router)
+    {
+        _metaGroup = metaGroup;
+        _streamManager = streamManager;
+        _consumerManager = consumerManager;
+        _router = router;
+    }
+
+    public static Task<MetaGroupTestFixture> StartAsync(int nodes)
+    {
+        var meta = new JetStreamMetaGroup(nodes);
+        var streamManager = new StreamManager(meta);
+        var consumerManager = new ConsumerManager(meta);
+        var router = new JetStreamApiRouter(streamManager, consumerManager, meta);
+        return Task.FromResult(new MetaGroupTestFixture(meta, streamManager, consumerManager, router));
+    }
+
+    public Task<NATS.Server.JetStream.Api.JetStreamApiResponse> CreateStreamAsync(string name, int replicas)
+    {
+        var response = _streamManager.CreateOrUpdate(new StreamConfig
+        {
+            Name = name,
+            Subjects = [name.ToLowerInvariant() + ".*"],
+            Replicas = replicas,
+        });
+        return Task.FromResult(response);
+    }
+
+    public Task<MetaGroupState> GetMetaStateAsync() => Task.FromResult(_metaGroup.GetState());
+
+    public Task<JetStreamApiResponse> RequestAsync(string subject, string payload)
+    {
+        return Task.FromResult(_router.Route(subject, System.Text.Encoding.UTF8.GetBytes(payload)));
+    }
+
+    public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+}