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/JetStreamApiProtocolIntegrationTests.cs

@@ -0,0 +1,68 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Client.Core;
+using NATS.Server.Configuration;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamApiProtocolIntegrationTests
+{
+    [Fact]
+    public async Task Js_api_request_over_pub_reply_returns_response_message()
+    {
+        await using var server = await ServerFixture.StartJetStreamEnabledAsync();
+        var response = await server.RequestAsync("$JS.API.INFO", "{}", timeoutMs: 1000);
+
+        response.ShouldContain("\"error\"");
+    }
+}
+
+internal sealed class ServerFixture : IAsyncDisposable
+{
+    private readonly NatsServer _server;
+    private readonly CancellationTokenSource _cts;
+
+    private ServerFixture(NatsServer server, CancellationTokenSource cts)
+    {
+        _server = server;
+        _cts = cts;
+    }
+
+    public static async Task<ServerFixture> StartJetStreamEnabledAsync()
+    {
+        var options = new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+            JetStream = new JetStreamOptions
+            {
+                StoreDir = Path.Combine(Path.GetTempPath(), $"nats-js-proto-{Guid.NewGuid():N}"),
+                MaxMemoryStore = 1024 * 1024,
+                MaxFileStore = 10 * 1024 * 1024,
+            },
+        };
+
+        var server = new NatsServer(options, NullLoggerFactory.Instance);
+        var cts = new CancellationTokenSource();
+        _ = server.StartAsync(cts.Token);
+        await server.WaitForReadyAsync();
+
+        return new ServerFixture(server, cts);
+    }
+
+    public async Task<string> RequestAsync(string subject, string payload, int timeoutMs)
+    {
+        await using var conn = new NatsConnection(new NatsOpts { Url = $"nats://127.0.0.1:{_server.Port}" });
+        await conn.ConnectAsync();
+
+        using var timeout = new CancellationTokenSource(TimeSpan.FromMilliseconds(timeoutMs));
+        var response = await conn.RequestAsync<string, string>(subject, payload, cancellationToken: timeout.Token);
+        return response.Data ?? string.Empty;
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _cts.CancelAsync();
+        _server.Dispose();
+        _cts.Dispose();
+    }
+}