feat: add MQTT MaxAckPending flow control (Gap 6.4)

Implements MqttFlowController with per-subscription SemaphoreSlim-based
slot tracking for QoS 1/2 messages. Replaces the previous catch-and-swallow
pattern with an explicit CurrentCount guard on Release. 10 unit tests added.

tests/NATS.Server.Tests/Mqtt/MqttQoSTrackingTests.cs

@@ -0,0 +1,135 @@
+// Tests for MqttQoS1Tracker (Gap 6.3 — JetStream-backed QoS 1/2 tracking).
+// Go reference: golang/nats-server/server/mqtt.go mqttProcessPub (~line 1200).
+
+using NATS.Server.Mqtt;
+using Shouldly;
+
+namespace NATS.Server.Tests.Mqtt;
+
+public sealed class MqttQoSTrackingTests
+{
+    // ── QoS 1 Tracker ────────────────────────────────────────────────────────
+
+    [Fact]
+    public void Register_assigns_packet_id()
+    {
+        // Go reference: server/mqtt.go mqttProcessPub — assigns non-zero packet ID for QoS 1
+        var tracker = new MqttQoS1Tracker();
+
+        var id = tracker.Register("sensors/temp", [0x01, 0x02]);
+
+        id.ShouldNotBe((ushort)0);
+    }
+
+    [Fact]
+    public void Register_increments_packet_id()
+    {
+        // Go reference: server/mqtt.go — each outgoing QoS 1 message gets a unique packet ID
+        var tracker = new MqttQoS1Tracker();
+
+        var id1 = tracker.Register("sensors/temp", [0x01]);
+        var id2 = tracker.Register("sensors/humidity", [0x02]);
+
+        id1.ShouldNotBe(id2);
+    }
+
+    [Fact]
+    public void Acknowledge_removes_pending()
+    {
+        // Go reference: server/mqtt.go mqttProcessPubAck — removes message from pending set
+        var tracker = new MqttQoS1Tracker();
+        var id = tracker.Register("sensors/temp", [0xAB]);
+
+        tracker.PendingCount.ShouldBe(1);
+        var removed = tracker.Acknowledge(id);
+
+        removed.ShouldBeTrue();
+        tracker.PendingCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public void Acknowledge_returns_false_for_unknown()
+    {
+        // Go reference: server/mqtt.go — PUBACK for unknown packet ID is silently ignored
+        var tracker = new MqttQoS1Tracker();
+
+        var result = tracker.Acknowledge(9999);
+
+        result.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void PendingCount_reflects_current_state()
+    {
+        // Register 3 messages, acknowledge 1, expect count of 2
+        var tracker = new MqttQoS1Tracker();
+        var id1 = tracker.Register("a/b", [1]);
+        tracker.Register("c/d", [2]);
+        tracker.Register("e/f", [3]);
+
+        tracker.Acknowledge(id1);
+
+        tracker.PendingCount.ShouldBe(2);
+    }
+
+    [Fact]
+    public void IsPending_true_for_registered()
+    {
+        // Go reference: server/mqtt.go — registered QoS 1 message is in the pending set
+        var tracker = new MqttQoS1Tracker();
+        var id = tracker.Register("topic/x", [0xFF]);
+
+        tracker.IsPending(id).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void IsPending_false_after_acknowledge()
+    {
+        // Go reference: server/mqtt.go — message is removed from pending after PUBACK
+        var tracker = new MqttQoS1Tracker();
+        var id = tracker.Register("topic/x", [0xFF]);
+        tracker.Acknowledge(id);
+
+        tracker.IsPending(id).ShouldBeFalse();
+    }
+
+    [Fact]
+    public void GetPendingForRedelivery_returns_all_pending()
+    {
+        // Go reference: server/mqtt.go reconnect path — all unacked messages are redelivered
+        var tracker = new MqttQoS1Tracker();
+        tracker.Register("a", [1]);
+        tracker.Register("b", [2]);
+        tracker.Register("c", [3]);
+
+        var pending = tracker.GetPendingForRedelivery();
+
+        pending.Count.ShouldBe(3);
+    }
+
+    [Fact]
+    public void GetPendingForRedelivery_increments_delivery_count()
+    {
+        // Go reference: server/mqtt.go reconnect redelivery — DUP flag set, delivery count increments
+        var tracker = new MqttQoS1Tracker();
+        tracker.Register("test/topic", [0xDE, 0xAD]);
+
+        var pending = tracker.GetPendingForRedelivery();
+
+        pending[0].DeliveryCount.ShouldBe(2);
+    }
+
+    [Fact]
+    public void Clear_removes_all_pending()
+    {
+        // Go reference: server/mqtt.go session cleanup — all pending messages discarded on clean session
+        var tracker = new MqttQoS1Tracker();
+        tracker.Register("x", [1]);
+        tracker.Register("y", [2]);
+        tracker.Register("z", [3]);
+
+        tracker.Clear();
+
+        tracker.PendingCount.ShouldBe(0);
+    }
+}