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/Mqtt/MqttBinaryDecoder.cs

@@ -193,9 +193,15 @@ public static class MqttBinaryDecoder
     /// Parses the payload bytes of an MQTT SUBSCRIBE packet.
     /// </summary>
     /// <param name="payload">The payload bytes from <see cref="MqttControlPacket.Payload"/>.</param>
+    /// <param name="flags">
+    /// Optional fixed-header flags nibble. When provided, must match SUBSCRIBE flags (0x02).
+    /// </param>
     /// <returns>A populated <see cref="MqttSubscribeInfo"/>.</returns>
-    public static MqttSubscribeInfo ParseSubscribe(ReadOnlySpan<byte> payload)
+    public static MqttSubscribeInfo ParseSubscribe(ReadOnlySpan<byte> payload, byte? flags = null)
     {
+        if (flags.HasValue && flags.Value != MqttProtocolConstants.SubscribeFlags)
+            throw new FormatException("MQTT SUBSCRIBE packet has invalid fixed-header flags.");
+
         // Variable header: packet identifier (2 bytes, big-endian)
         // Payload: one or more topic-filter entries, each:
         //   2-byte length prefix + UTF-8 filter string + 1-byte requested QoS