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/Monitoring/MonitoringHealthAndSortParityBatch1Tests.cs

@@ -0,0 +1,39 @@
+using System.Text.Json;
+using NATS.Server.Monitoring;
+
+namespace NATS.Server.Tests.Monitoring;
+
+public class MonitoringHealthAndSortParityBatch1Tests
+{
+    [Fact]
+    public void SortOpt_IsValid_matches_defined_values()
+    {
+        foreach (var value in Enum.GetValues<SortOpt>())
+            value.IsValid().ShouldBeTrue();
+
+        ((SortOpt)999).IsValid().ShouldBeFalse();
+    }
+
+    [Fact]
+    public void HealthStatus_ok_serializes_with_go_shape_fields()
+    {
+        var json = JsonSerializer.Serialize(HealthStatus.Ok());
+
+        json.ShouldContain("\"status\":\"ok\"");
+        json.ShouldContain("\"status_code\":200");
+        json.ShouldContain("\"errors\":[]");
+    }
+
+    [Fact]
+    public void HealthzError_serializes_enum_as_string()
+    {
+        var json = JsonSerializer.Serialize(new HealthzError
+        {
+            Type = HealthzErrorType.JetStream,
+            Error = "jetstream unavailable",
+        });
+
+        json.ShouldContain("\"type\":\"JetStream\"");
+        json.ShouldContain("\"error\":\"jetstream unavailable\"");
+    }
+}