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)));
+    }
+}

tests/NATS.Server.Tests/JetStreamApiRouterTests.cs

@@ -0,0 +1,12 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamApiRouterTests
+{
+    [Fact]
+    public async Task Unknown_js_api_subject_returns_structured_error()
+    {
+        var response = await JetStreamApiFixture.RequestAsync("$JS.API.BAD", "{}");
+        response.Error.ShouldNotBeNull();
+        response.Error!.Code.ShouldBe(404);
+    }
+}