feat: add advisory event publication for API operations (Gap 7.6)

Add JetStream advisory subject constants to EventSubjects and a lightweight
AdvisoryPublisher that publishes stream/consumer lifecycle events to
$JS.EVENT.ADVISORY.* subjects without depending on InternalEventSystem
directly (testable via Action delegate injection).

src/NATS.Server/JetStream/Api/AdvisoryPublisher.cs

@@ -0,0 +1,130 @@
+namespace NATS.Server.JetStream.Api;
+
+/// <summary>
+/// Publishes JetStream advisory events to $JS.EVENT.ADVISORY.* subjects.
+/// Designed to be lightweight and testable; accepts a publish action delegate
+/// rather than depending directly on InternalEventSystem.
+/// Go reference: jetstream_api.go advisory publication.
+/// </summary>
+public sealed class AdvisoryPublisher
+{
+    private readonly Action<string, object> _publishAction;
+    private long _publishCount;
+
+    public AdvisoryPublisher(Action<string, object> publishAction)
+    {
+        _publishAction = publishAction;
+    }
+
+    /// <summary>
+    /// Total number of advisory events published since creation.
+    /// </summary>
+    public long PublishCount => Interlocked.Read(ref _publishCount);
+
+    /// <summary>
+    /// Publishes a stream created advisory.
+    /// Go reference: jetstream_api.go — advisory on stream creation.
+    /// </summary>
+    public void StreamCreated(string streamName, object? detail = null)
+    {
+        var subject = string.Format(Events.EventSubjects.JsAdvisoryStreamCreated, streamName);
+        Publish(subject, new AdvisoryEvent
+        {
+            Type = "io.nats.jetstream.advisory.stream_created",
+            Stream = streamName,
+            TimeStamp = DateTime.UtcNow,
+            Detail = detail,
+        });
+    }
+
+    /// <summary>
+    /// Publishes a stream deleted advisory.
+    /// Go reference: jetstream_api.go — advisory on stream deletion.
+    /// </summary>
+    public void StreamDeleted(string streamName)
+    {
+        var subject = string.Format(Events.EventSubjects.JsAdvisoryStreamDeleted, streamName);
+        Publish(subject, new AdvisoryEvent
+        {
+            Type = "io.nats.jetstream.advisory.stream_deleted",
+            Stream = streamName,
+            TimeStamp = DateTime.UtcNow,
+        });
+    }
+
+    /// <summary>
+    /// Publishes a stream updated advisory.
+    /// Go reference: jetstream_api.go — advisory on stream config update.
+    /// </summary>
+    public void StreamUpdated(string streamName, object? detail = null)
+    {
+        var subject = string.Format(Events.EventSubjects.JsAdvisoryStreamUpdated, streamName);
+        Publish(subject, new AdvisoryEvent
+        {
+            Type = "io.nats.jetstream.advisory.stream_updated",
+            Stream = streamName,
+            TimeStamp = DateTime.UtcNow,
+            Detail = detail,
+        });
+    }
+
+    /// <summary>
+    /// Publishes a consumer created advisory.
+    /// Go reference: jetstream_api.go — advisory on consumer creation.
+    /// </summary>
+    public void ConsumerCreated(string streamName, string consumerName)
+    {
+        var subject = string.Format(Events.EventSubjects.JsAdvisoryConsumerCreated, streamName, consumerName);
+        Publish(subject, new AdvisoryEvent
+        {
+            Type = "io.nats.jetstream.advisory.consumer_created",
+            Stream = streamName,
+            Consumer = consumerName,
+            TimeStamp = DateTime.UtcNow,
+        });
+    }
+
+    /// <summary>
+    /// Publishes a consumer deleted advisory.
+    /// Go reference: jetstream_api.go — advisory on consumer deletion.
+    /// </summary>
+    public void ConsumerDeleted(string streamName, string consumerName)
+    {
+        var subject = string.Format(Events.EventSubjects.JsAdvisoryConsumerDeleted, streamName, consumerName);
+        Publish(subject, new AdvisoryEvent
+        {
+            Type = "io.nats.jetstream.advisory.consumer_deleted",
+            Stream = streamName,
+            Consumer = consumerName,
+            TimeStamp = DateTime.UtcNow,
+        });
+    }
+
+    private void Publish(string subject, AdvisoryEvent evt)
+    {
+        Interlocked.Increment(ref _publishCount);
+        _publishAction(subject, evt);
+    }
+}
+
+/// <summary>
+/// Advisory event payload describing a JetStream lifecycle event.
+/// Go reference: jetstream_api.go advisory event types.
+/// </summary>
+public sealed class AdvisoryEvent
+{
+    /// <summary>Reverse-DNS event type identifier (e.g., "io.nats.jetstream.advisory.stream_created").</summary>
+    public string Type { get; init; } = string.Empty;
+
+    /// <summary>Name of the stream involved in the event, if applicable.</summary>
+    public string? Stream { get; init; }
+
+    /// <summary>Name of the consumer involved in the event, if applicable.</summary>
+    public string? Consumer { get; init; }
+
+    /// <summary>UTC timestamp when the advisory was generated.</summary>
+    public DateTime TimeStamp { get; init; }
+
+    /// <summary>Optional additional detail payload (arbitrary object).</summary>
+    public object? Detail { get; init; }
+}