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/MqttPacketWriter.cs

@@ -1,7 +1,24 @@
+using System.Buffers.Binary;
+using System.Text;
+
 namespace NATS.Server.Mqtt;
 
 public static class MqttPacketWriter
 {
+    public static byte[] WriteString(string value)
+        => WriteBytes(Encoding.UTF8.GetBytes(value));
+
+    public static byte[] WriteBytes(ReadOnlySpan<byte> bytes)
+    {
+        if (bytes.Length > ushort.MaxValue)
+            throw new ArgumentOutOfRangeException(nameof(bytes), "MQTT length-prefixed field cannot exceed 65535 bytes.");
+
+        var buffer = new byte[2 + bytes.Length];
+        BinaryPrimitives.WriteUInt16BigEndian(buffer.AsSpan(0, 2), (ushort)bytes.Length);
+        bytes.CopyTo(buffer.AsSpan(2));
+        return buffer;
+    }
+
     public static byte[] Write(MqttControlPacketType type, ReadOnlySpan<byte> payload, byte flags = 0)
     {
         if (type == MqttControlPacketType.Reserved)
@@ -18,8 +35,10 @@ public static class MqttPacketWriter
 
     internal static byte[] EncodeRemainingLength(int value)
     {
-        if (value < 0 || value > 268_435_455)
-            throw new ArgumentOutOfRangeException(nameof(value), "MQTT remaining length must be between 0 and 268435455.");
+        if (value < 0 || value > MqttProtocolConstants.MaxPayloadSize)
+            throw new ArgumentOutOfRangeException(
+                nameof(value),
+                $"MQTT remaining length must be between 0 and {MqttProtocolConstants.MaxPayloadSize}.");
 
         Span<byte> scratch = stackalloc byte[4];
         var index = 0;