feat: wire snapshot/restore API endpoints (Gap 7.4 stub)

Add HandleSnapshotAsync and HandleRestoreAsync with stream-name validation,
chunk metadata (NumChunks, BlkSize) in the response, and richer error codes.
Add StreamManager.Exists helper. Add JetStreamSnapshot.StreamName/NumChunks/BlkSize
fields. Fix AdvisoryEventTests.cs using-directive ordering. Add 12 SnapshotApiTests.

src/NATS.Server/JetStream/Api/Handlers/StreamApiHandlers.cs

@@ -189,6 +189,66 @@ public static class StreamApiHandlers
             : JetStreamApiResponse.NotFound(subject);
     }
 
+    /// <summary>
+    /// Async snapshot handler that validates stream existence before creating the snapshot,
+    /// and enriches the response with stream name and chunk metadata.
+    /// Go reference: server/jetstream_api.go — jsStreamSnapshotT handler.
+    /// </summary>
+    public static async Task<JetStreamApiResponse> HandleSnapshotAsync(
+        string subject,
+        StreamManager streamManager,
+        CancellationToken ct)
+    {
+        _ = ct;
+
+        var streamName = ExtractTrailingToken(subject, SnapshotPrefix);
+        if (streamName == null)
+            return JetStreamApiResponse.NotFound(subject);
+
+        if (!streamManager.Exists(streamName))
+            return JetStreamApiResponse.NotFound(subject);
+
+        var snapshot = streamManager.CreateSnapshot(streamName);
+        if (snapshot == null)
+            return JetStreamApiResponse.ErrorResponse(500, "snapshot creation failed");
+
+        return new JetStreamApiResponse
+        {
+            Snapshot = new JetStreamSnapshot
+            {
+                Payload = Convert.ToBase64String(snapshot),
+                StreamName = streamName,
+                NumChunks = 1,
+                BlkSize = snapshot.Length,
+            },
+        };
+    }
+
+    /// <summary>
+    /// Async restore handler that validates the payload and returns a structured error on failure.
+    /// Go reference: server/jetstream_api.go — jsStreamRestoreT handler.
+    /// </summary>
+    public static async Task<JetStreamApiResponse> HandleRestoreAsync(
+        string subject,
+        byte[] payload,
+        StreamManager streamManager,
+        CancellationToken ct)
+    {
+        _ = ct;
+
+        var streamName = ExtractTrailingToken(subject, RestorePrefix);
+        if (streamName == null)
+            return JetStreamApiResponse.NotFound(subject);
+
+        var snapshotBytes = ParseRestorePayload(payload);
+        if (snapshotBytes == null)
+            return JetStreamApiResponse.ErrorResponse(400, "snapshot payload required");
+
+        return streamManager.RestoreSnapshot(streamName, snapshotBytes)
+            ? JetStreamApiResponse.SuccessResponse()
+            : JetStreamApiResponse.ErrorResponse(500, "restore failed");
+    }
+
     // ---------------------------------------------------------------
     // Clustered handlers — propose to meta RAFT group instead of local StreamManager.
     // Go reference: jetstream_cluster.go:7620-7900 jsClusteredStreamRequest and related.

src/NATS.Server/JetStream/StreamManager.cs

@@ -115,6 +115,8 @@ public sealed class StreamManager
 
     public bool TryGet(string name, out StreamHandle handle) => _streams.TryGetValue(name, out handle!);
 
+    public bool Exists(string name) => _streams.ContainsKey(name);
+
     public bool Delete(string name)
     {
         if (!_streams.TryRemove(name, out _))