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/Auth/AuthModelAndCalloutConstantsParityTests.cs

@@ -0,0 +1,46 @@
+using NATS.Server.Auth;
+
+namespace NATS.Server.Tests.Auth;
+
+public class AuthModelAndCalloutConstantsParityTests
+{
+    [Fact]
+    public void NkeyUser_exposes_parity_fields()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var nkeyUser = new NKeyUser
+        {
+            Nkey = "UABC",
+            Issued = now,
+            AllowedConnectionTypes = new HashSet<string> { "STANDARD", "WEBSOCKET" },
+            ProxyRequired = true,
+        };
+
+        nkeyUser.Issued.ShouldBe(now);
+        nkeyUser.ProxyRequired.ShouldBeTrue();
+        nkeyUser.AllowedConnectionTypes.ShouldContain("STANDARD");
+    }
+
+    [Fact]
+    public void User_exposes_parity_fields()
+    {
+        var user = new User
+        {
+            Username = "alice",
+            Password = "secret",
+            AllowedConnectionTypes = new HashSet<string> { "STANDARD" },
+            ProxyRequired = false,
+        };
+
+        user.ProxyRequired.ShouldBeFalse();
+        user.AllowedConnectionTypes.ShouldContain("STANDARD");
+    }
+
+    [Fact]
+    public void External_auth_callout_constants_match_go_subjects_and_header()
+    {
+        ExternalAuthCalloutAuthenticator.AuthCalloutSubject.ShouldBe("$SYS.REQ.USER.AUTH");
+        ExternalAuthCalloutAuthenticator.AuthRequestSubject.ShouldBe("nats-authorization-request");
+        ExternalAuthCalloutAuthenticator.AuthRequestXKeyHeader.ShouldBe("Nats-Server-Xkey");
+    }
+}