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/NatsServerFixture.cs

@@ -0,0 +1,31 @@
+using NATS.Client.Core;
+
+namespace NATS.E2E.Tests.Infrastructure;
+
+/// <summary>
+/// xUnit fixture that manages a single NATS server process shared across a test collection.
+/// </summary>
+public sealed class NatsServerFixture : IAsyncLifetime
+{
+    private NatsServerProcess _server = null!;
+
+    public int Port => _server.Port;
+
+    public string ServerOutput => _server.Output;
+
+    public async Task InitializeAsync()
+    {
+        _server = new NatsServerProcess();
+        await _server.StartAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _server.DisposeAsync();
+    }
+
+    public NatsConnection CreateClient()
+    {
+        return new NatsConnection(new NatsOpts { Url = $"nats://127.0.0.1:{Port}" });
+    }
+}