feat: add slow consumer per-kind tracking with account counters (Gap 5.5)

Adds SlowConsumerTracker class for per-ClientKind slow consumer counting
with configurable threshold callbacks, and extends Account with atomic
IncrementSlowConsumers/SlowConsumerCount/ResetSlowConsumerCount members.
Includes 10 unit tests covering concurrency, threshold firing, and reset.

src/NATS.Server/Auth/Account.cs

@@ -152,6 +152,16 @@ public sealed class Account : IDisposable
         return true;
     }
 
+    // Slow consumer tracking
+    // Go reference: server/client.go — handleSlowConsumer, markConnAsSlow, server/accounts.go slowConsumerCount
+    private long _slowConsumerCount;
+
+    public long SlowConsumerCount => Interlocked.Read(ref _slowConsumerCount);
+
+    public void IncrementSlowConsumers() => Interlocked.Increment(ref _slowConsumerCount);
+
+    public void ResetSlowConsumerCount() => Interlocked.Exchange(ref _slowConsumerCount, 0L);
+
     // Per-account message/byte stats
     private long _inMsgs;
     private long _outMsgs;