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/AccountServerFixture.cs

@@ -0,0 +1,49 @@
+using NATS.Client.Core;
+
+namespace NATS.E2E.Tests.Infrastructure;
+
+public sealed class AccountServerFixture : IAsyncLifetime
+{
+    private NatsServerProcess _server = null!;
+
+    public int Port => _server.Port;
+
+    public async Task InitializeAsync()
+    {
+        var config = """
+            accounts {
+              ACCT_A {
+                users = [
+                  { user: "user_a", password: "pass_a" }
+                ]
+              }
+              ACCT_B {
+                users = [
+                  { user: "user_b", password: "pass_b" }
+                ]
+              }
+            }
+            """;
+
+        _server = NatsServerProcess.WithConfig(config);
+        await _server.StartAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _server.DisposeAsync();
+    }
+
+    public NatsConnection CreateClientA()
+    {
+        return new NatsConnection(new NatsOpts { Url = $"nats://user_a:pass_a@127.0.0.1:{Port}" });
+    }
+
+    public NatsConnection CreateClientB()
+    {
+        return new NatsConnection(new NatsOpts { Url = $"nats://user_b:pass_b@127.0.0.1:{Port}" });
+    }
+}
+
+[CollectionDefinition("E2E-Accounts")]
+public class AccountsCollection : ICollectionFixture<AccountServerFixture>;