Fix E2E test gaps and add comprehensive E2E + parity test suites

- Fix pull consumer fetch: send original stream subject in HMSG (not inbox)
  so NATS client distinguishes data messages from control messages
- Fix MaxAge expiry: add background timer in StreamManager for periodic pruning
- Fix JetStream wire format: Go-compatible anonymous objects with string enums,
  proper offset-based pagination for stream/consumer list APIs
- Add 42 E2E black-box tests (core messaging, auth, TLS, accounts, JetStream)
- Add ~1000 parity tests across all subsystems (gaps closure)
- Update gap inventory docs to reflect implementation status

tests/NATS.Server.Tests/Gateways/GatewayServerAccessorParityBatch4Tests.cs

@@ -0,0 +1,37 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+
+namespace NATS.Server.Tests.Gateways;
+
+public class GatewayServerAccessorParityBatch4Tests
+{
+    [Fact]
+    public void Gateway_address_url_and_name_accessors_reflect_gateway_options()
+    {
+        using var server = new NatsServer(
+            new NatsOptions
+            {
+                Gateway = new GatewayOptions
+                {
+                    Name = "gw-a",
+                    Host = "127.0.0.1",
+                    Port = 7222,
+                },
+            },
+            NullLoggerFactory.Instance);
+
+        server.GatewayAddr().ShouldBe("127.0.0.1:7222");
+        server.GetGatewayURL().ShouldBe("127.0.0.1:7222");
+        server.GetGatewayName().ShouldBe("gw-a");
+    }
+
+    [Fact]
+    public void Gateway_accessors_return_null_when_gateway_is_not_configured()
+    {
+        using var server = new NatsServer(new NatsOptions(), NullLoggerFactory.Instance);
+
+        server.GatewayAddr().ShouldBeNull();
+        server.GetGatewayURL().ShouldBeNull();
+        server.GetGatewayName().ShouldBeNull();
+    }
+}