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/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 _))