feat(auth): add account import/export cycle detection and JetStream limits (E4+E5)

E4: AccountImportExport with DFS cycle detection for service imports,
RemoveServiceImport/RemoveStreamImport, and ValidateImport authorization.
E5: AccountLimits record with MaxStorage/MaxConsumers/MaxAckPending,
TryReserveConsumer/ReleaseConsumer, TrackStorageDelta on Account.
20 new tests, all passing.

src/NATS.Server/Auth/AccountLimits.cs

@@ -0,0 +1,32 @@
+// Per-account JetStream resource limits.
+// Go reference: accounts.go JetStreamAccountLimits struct.
+
+namespace NATS.Server.Auth;
+
+/// <summary>
+/// Per-account limits on JetStream resources: storage, streams, consumers, and ack pending.
+/// A value of 0 means unlimited for all fields.
+/// </summary>
+public sealed record AccountLimits
+{
+    /// <summary>Maximum total storage in bytes (0 = unlimited).</summary>
+    public long MaxStorage { get; init; }
+
+    /// <summary>Maximum number of streams (0 = unlimited).</summary>
+    public int MaxStreams { get; init; }
+
+    /// <summary>Maximum number of consumers (0 = unlimited).</summary>
+    public int MaxConsumers { get; init; }
+
+    /// <summary>Maximum pending ack count per consumer (0 = unlimited).</summary>
+    public int MaxAckPending { get; init; }
+
+    /// <summary>Maximum memory-based storage in bytes (0 = unlimited).</summary>
+    public long MaxMemoryStorage { get; init; }
+
+    /// <summary>Maximum disk-based storage in bytes (0 = unlimited).</summary>
+    public long MaxDiskStorage { get; init; }
+
+    /// <summary>Default instance with all limits set to unlimited (0).</summary>
+    public static AccountLimits Unlimited { get; } = new();
+}