fix(test): use precise InvalidDataException assertion in wrong-key encryption test

Code quality review feedback: Should.Throw<Exception> was too broad,
changed to Should.Throw<InvalidDataException> to match the actual
exception type thrown when AEAD decryption fails with wrong key.

tests/NATS.Server.Tests/FileStoreEncryptionTests.cs

@@ -92,13 +92,13 @@ public class FileStoreEncryptionTests
             await store.AppendAsync("secret", "data"u8.ToArray(), default);
         }
 
-        // Recovery with wrong key should throw or return no messages
+        // Recovery with wrong key should throw InvalidDataException (from CryptographicException)
         var act = () => new FileStore(new FileStoreOptions
         {
             Directory = dir.FullName,
             Cipher = StoreCipher.ChaCha,
             EncryptionKey = key2,
         });
-        Should.Throw<Exception>(act);
+        Should.Throw<InvalidDataException>(act);
     }
 }