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

src/NATS.Server/JetStream/Api/JetStreamApiLimits.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.JetStream.Api;
+
+/// <summary>
+/// JetStream API size and queue limits aligned with Go server constants.
+/// Go reference: server/jetstream_api.go (JSMaxDescriptionLen, JSMaxMetadataLen,
+/// JSMaxNameLen, JSDefaultRequestQueueLimit).
+/// </summary>
+public static class JetStreamApiLimits
+{
+    public const int JSMaxDescriptionLen = 4_096;
+    public const int JSMaxMetadataLen = 128 * 1024;
+    public const int JSMaxNameLen = 255;
+    public const int JSDefaultRequestQueueLimit = 10_000;
+}