feat(consumer): add priority group pin ID management

Add AssignPinId, ValidatePinId, and UnassignPinId to PriorityGroupManager,
plus CurrentPinId tracking on PriorityGroup, porting Go consumer.go
(setPinnedTimer, assignNewPinId) pin ID semantics. Covered by 7 new tests.

src/NATS.Server/JetStream/Consumers/PriorityGroupManager.cs

@@ -92,10 +92,61 @@ public sealed class PriorityGroupManager
         return active != null && string.Equals(active, consumerId, StringComparison.Ordinal);
     }
 
+    /// <summary>
+    /// Assign a new pin ID to the named group, replacing any existing pin.
+    /// Go reference: consumer.go (assignNewPinId).
+    /// </summary>
+    /// <returns>The newly generated 22-character pin ID.</returns>
+    public string AssignPinId(string groupName, string consumerId)
+    {
+        if (!_groups.TryGetValue(groupName, out var group))
+            return string.Empty;
+
+        var pinId = Guid.NewGuid().ToString("N")[..22];
+        lock (group.Lock)
+        {
+            group.CurrentPinId = pinId;
+        }
+
+        return pinId;
+    }
+
+    /// <summary>
+    /// Returns <c>true</c> if the group exists and its current pin ID equals <paramref name="pinId"/>.
+    /// Go reference: consumer.go (setPinnedTimer).
+    /// </summary>
+    public bool ValidatePinId(string groupName, string pinId)
+    {
+        if (!_groups.TryGetValue(groupName, out var group))
+            return false;
+
+        lock (group.Lock)
+        {
+            return group.CurrentPinId != null &&
+                   string.Equals(group.CurrentPinId, pinId, StringComparison.Ordinal);
+        }
+    }
+
+    /// <summary>
+    /// Clear the current pin ID for the named group. No-op if the group does not exist.
+    /// Go reference: consumer.go (setPinnedTimer).
+    /// </summary>
+    public void UnassignPinId(string groupName)
+    {
+        if (!_groups.TryGetValue(groupName, out var group))
+            return;
+
+        lock (group.Lock)
+        {
+            group.CurrentPinId = null;
+        }
+    }
+
     private sealed class PriorityGroup
     {
         public object Lock { get; } = new();
         public List<PriorityMember> Members { get; } = [];
+        public string? CurrentPinId { get; set; }
     }
 
     private record struct PriorityMember(string ConsumerId, int Priority);