test: lock FileStore optimization boundaries

tests/NATS.Server.JetStream.Tests/JetStream/Storage/StoreInterfaceTests.cs

@@ -532,4 +532,22 @@ public sealed class StoreInterfaceTests
         lastMsg = s.LoadLastMsg("foo", null);
         lastMsg.Sequence.ShouldBe(2UL);
     }
+
+    [Fact]
+    public void FileStore_LoadMsg_preserves_headers_separately_from_payload()
+    {
+        var dir = Directory.CreateTempSubdirectory();
+        using var store = new FileStore(new FileStoreOptions { Directory = dir.FullName });
+
+        var hdr = "NATS/1.0\r\nX-Test: one\r\n\r\n"u8.ToArray();
+        var msg = "payload"u8.ToArray();
+
+        var (seq, _) = store.StoreMsg("foo", hdr, msg, 0L);
+        var loaded = store.LoadMsg(seq, null);
+
+        loaded.Header.ShouldNotBeNull();
+        loaded.Header.ShouldBe(hdr);
+        loaded.Data.ShouldNotBeNull();
+        loaded.Data.ShouldBe(msg);
+    }
 }