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/MqttParityModels.cs

@@ -0,0 +1,90 @@
+namespace NATS.Server.Mqtt;
+
+/// <summary>
+/// JetStream API helper context for MQTT account/session operations.
+/// Go reference: mqtt.go mqttJSA.
+/// </summary>
+public sealed class MqttJsa
+{
+    public string AccountName { get; set; } = string.Empty;
+    public string ReplyPrefix { get; set; } = string.Empty;
+    public string? Domain { get; set; }
+}
+
+/// <summary>
+/// MQTT JetStream publish request shape.
+/// Go reference: mqtt.go mqttJSPubMsg.
+/// </summary>
+public sealed class MqttJsPubMsg
+{
+    public string Subject { get; set; } = string.Empty;
+    public byte[] Payload { get; set; } = [];
+    public string? ReplyTo { get; set; }
+}
+
+/// <summary>
+/// Retained-message delete notification payload.
+/// Go reference: mqtt.go mqttRetMsgDel.
+/// </summary>
+public sealed class MqttRetMsgDel
+{
+    public string Topic { get; set; } = string.Empty;
+    public ulong Sequence { get; set; }
+}
+
+/// <summary>
+/// Persisted MQTT session metadata.
+/// Go reference: mqtt.go mqttPersistedSession.
+/// </summary>
+public sealed class MqttPersistedSession
+{
+    public string ClientId { get; set; } = string.Empty;
+    public int LastPacketId { get; set; }
+    public int MaxAckPending { get; set; }
+}
+
+/// <summary>
+/// Reference to a retained message in storage.
+/// Go reference: mqtt.go mqttRetainedMsgRef.
+/// </summary>
+public sealed class MqttRetainedMessageRef
+{
+    public ulong StreamSequence { get; set; }
+    public string Subject { get; set; } = string.Empty;
+}
+
+/// <summary>
+/// MQTT subscription metadata.
+/// Go reference: mqtt.go mqttSub.
+/// </summary>
+public sealed class MqttSub
+{
+    public string Filter { get; set; } = string.Empty;
+    public byte Qos { get; set; }
+    public string? JsDur { get; set; }
+    public bool Prm { get; set; }
+    public bool Reserved { get; set; }
+}
+
+/// <summary>
+/// Parsed MQTT filter metadata.
+/// Go reference: mqtt.go mqttFilter.
+/// </summary>
+public sealed class MqttFilter
+{
+    public string Filter { get; set; } = string.Empty;
+    public byte Qos { get; set; }
+    public string? TopicToken { get; set; }
+}
+
+/// <summary>
+/// Parsed NATS headers associated with an MQTT publish flow.
+/// Go reference: mqtt.go mqttParsedPublishNATSHeader.
+/// </summary>
+public sealed class MqttParsedPublishNatsHeader
+{
+    public string? Subject { get; set; }
+    public string? Mapped { get; set; }
+    public bool IsPublish { get; set; }
+    public bool IsPubRel { get; set; }
+}