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/JetStreamParityModels.cs

@@ -0,0 +1,58 @@
+namespace NATS.Server.JetStream;
+
+/// <summary>
+/// API usage counters for JetStream.
+/// Go reference: server/jetstream.go JetStreamAPIStats.
+/// </summary>
+public sealed class JetStreamApiStats
+{
+    public int Level { get; set; }
+    public ulong Total { get; set; }
+    public ulong Errors { get; set; }
+    public int Inflight { get; set; }
+}
+
+/// <summary>
+/// Per-tier JetStream resource view.
+/// Go reference: server/jetstream.go JetStreamTier.
+/// </summary>
+public sealed class JetStreamTier
+{
+    public string Name { get; set; } = string.Empty;
+    public long Memory { get; set; }
+    public long Store { get; set; }
+    public int Streams { get; set; }
+    public int Consumers { get; set; }
+}
+
+/// <summary>
+/// Per-account JetStream limits.
+/// Go reference: server/jetstream.go JetStreamAccountLimits.
+/// </summary>
+public sealed class JetStreamAccountLimits
+{
+    public long MaxMemory { get; set; }
+    public long MaxStore { get; set; }
+    public int MaxStreams { get; set; }
+    public int MaxConsumers { get; set; }
+    public int MaxAckPending { get; set; }
+    public long MemoryMaxStreamBytes { get; set; }
+    public long StoreMaxStreamBytes { get; set; }
+    public bool MaxBytesRequired { get; set; }
+    public Dictionary<string, JetStreamTier> Tiers { get; set; } = new(StringComparer.Ordinal);
+}
+
+/// <summary>
+/// Server-level JetStream usage stats.
+/// Go reference: server/jetstream.go JetStreamStats.
+/// </summary>
+public sealed class JetStreamStats
+{
+    public long Memory { get; set; }
+    public long Store { get; set; }
+    public long ReservedMemory { get; set; }
+    public long ReservedStore { get; set; }
+    public int Accounts { get; set; }
+    public int HaAssets { get; set; }
+    public JetStreamApiStats Api { get; set; } = new();
+}