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.E2E.Tests/Infrastructure/E2ETestHelper.cs

@@ -0,0 +1,15 @@
+using NATS.Client.Core;
+
+namespace NATS.E2E.Tests.Infrastructure;
+
+public static class E2ETestHelper
+{
+    public static NatsConnection CreateClient(int port)
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{port}" });
+
+    public static NatsConnection CreateClient(int port, NatsOpts opts)
+        => new(opts with { Url = $"nats://127.0.0.1:{port}" });
+
+    public static CancellationToken Timeout(int seconds = 10)
+        => new CancellationTokenSource(TimeSpan.FromSeconds(seconds)).Token;
+}