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/SlowConsumerTracker.cs

@@ -0,0 +1,59 @@
+using System.Collections.Concurrent;
+
+namespace NATS.Server;
+
+/// <summary>
+/// Tracks slow consumer events per <see cref="ClientKind"/> with optional threshold-based callbacks.
+/// Go reference: server/client.go — handleSlowConsumer, markConnAsSlow.
+/// </summary>
+public sealed class SlowConsumerTracker
+{
+    private readonly ConcurrentDictionary<ClientKind, long> _countsByKind = new();
+    private long _totalCount;
+    private readonly int _threshold;
+    private Action<ClientKind>? _onThresholdExceeded;
+
+    /// <param name="threshold">
+    /// When the total count reaches this value the registered callback is fired.
+    /// 0 means no threshold (callback never fires automatically).
+    /// </param>
+    public SlowConsumerTracker(int threshold = 0)
+    {
+        _threshold = threshold;
+    }
+
+    /// <summary>Total slow-consumer events recorded across all <see cref="ClientKind"/>s.</summary>
+    public long TotalCount => Interlocked.Read(ref _totalCount);
+
+    /// <summary>
+    /// Records one slow-consumer event for the given <paramref name="kind"/>.
+    /// Increments both the per-kind counter and the total counter.
+    /// Fires the threshold callback if <see cref="TotalCount"/> reaches the configured threshold.
+    /// </summary>
+    public void RecordSlowConsumer(ClientKind kind)
+    {
+        _countsByKind.AddOrUpdate(kind, 1L, static (_, existing) => existing + 1L);
+        var total = Interlocked.Increment(ref _totalCount);
+
+        if (_threshold > 0 && total >= _threshold)
+            _onThresholdExceeded?.Invoke(kind);
+    }
+
+    /// <summary>Returns the number of slow-consumer events recorded for <paramref name="kind"/>.</summary>
+    public long GetCount(ClientKind kind) =>
+        _countsByKind.TryGetValue(kind, out var count) ? count : 0L;
+
+    /// <summary>
+    /// Registers a callback that is invoked (with the triggering <see cref="ClientKind"/>)
+    /// each time the total count reaches or exceeds the configured threshold.
+    /// </summary>
+    public void OnThresholdExceeded(Action<ClientKind> callback) =>
+        _onThresholdExceeded = callback;
+
+    /// <summary>Resets all per-kind and total counters to zero.</summary>
+    public void Reset()
+    {
+        _countsByKind.Clear();
+        Interlocked.Exchange(ref _totalCount, 0L);
+    }
+}