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/BasicTests.cs

@@ -0,0 +1,67 @@
+using NATS.Client.Core;
+using NATS.E2E.Tests.Infrastructure;
+
+namespace NATS.E2E.Tests;
+
+[Collection("E2E")]
+public class BasicTests(NatsServerFixture fixture)
+{
+    [Fact]
+    public async Task ConnectAndPing()
+    {
+        await using var client = fixture.CreateClient();
+        await client.ConnectAsync();
+
+        await client.PingAsync();
+
+        client.ConnectionState.ShouldBe(NatsConnectionState.Open);
+    }
+
+    [Fact]
+    public async Task PubSub()
+    {
+        await using var pub = fixture.CreateClient();
+        await using var sub = fixture.CreateClient();
+
+        await pub.ConnectAsync();
+        await sub.ConnectAsync();
+
+        await using var subscription = await sub.SubscribeCoreAsync<string>("e2e.test.pubsub");
+        await sub.PingAsync(); // Flush to ensure subscription is active
+
+        await pub.PublishAsync("e2e.test.pubsub", "hello e2e");
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+        var msg = await subscription.Msgs.ReadAsync(cts.Token);
+
+        msg.Data.ShouldBe("hello e2e");
+    }
+
+    [Fact]
+    public async Task RequestReply()
+    {
+        await using var requester = fixture.CreateClient();
+        await using var responder = fixture.CreateClient();
+
+        await requester.ConnectAsync();
+        await responder.ConnectAsync();
+
+        await using var subscription = await responder.SubscribeCoreAsync<string>("e2e.test.rpc");
+        await responder.PingAsync(); // Flush to ensure subscription is active
+
+        // Background task to read and reply
+        var responderTask = Task.Run(async () =>
+        {
+            using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+            var msg = await subscription.Msgs.ReadAsync(cts.Token);
+            await responder.PublishAsync(msg.ReplyTo!, $"reply: {msg.Data}");
+        });
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+        var reply = await requester.RequestAsync<string, string>("e2e.test.rpc", "ping", cancellationToken: cts.Token);
+
+        reply.Data.ShouldBe("reply: ping");
+
+        await responderTask; // Ensure no exceptions in the responder
+    }
+}