feat: add jetstream api router and error envelope

tests/NATS.Server.Tests/JetStreamApiFixture.cs

@@ -0,0 +1,14 @@
+using System.Text;
+using NATS.Server.JetStream.Api;
+
+namespace NATS.Server.Tests;
+
+internal static class JetStreamApiFixture
+{
+    private static readonly JetStreamApiRouter Router = new();
+
+    public static Task<JetStreamApiResponse> RequestAsync(string subject, string payload)
+    {
+        return Task.FromResult(Router.Route(subject, Encoding.UTF8.GetBytes(payload)));
+    }
+}