feat: add JetStream storage recovery & encryption tests (Task 6)

Port Go jetstream_test.go storage/recovery/encryption tests. Add stream
recovery stubs, encryption key management, direct-get with time queries,
and subject delete marker handling.

23 new tests ported from jetstream_test.go.

src/NATS.Server/JetStream/StreamManager.cs

@@ -619,7 +619,9 @@ public sealed class StreamManager
                 Directory = Path.Combine(Path.GetTempPath(), "natsdotnet-js-store", config.Name),
                 MaxAgeMs = config.MaxAgeMs,
             }),
-            _ => new MemStore(),
+            // Go: newMemStore — pass full config so FirstSeq, MaxMsgsPer, AllowMsgTtl, etc. apply.
+            // Reference: server/memstore.go:99 (newMemStore constructor).
+            _ => new MemStore(config),
         };
     }
 }