fix(auth): C4 review polish — document backward-compat JSON tolerance, shared BundleJsonOptions, PreviewAsync legacy-bundle test, doc fix (review I-2/I-3/M-1/M-2; I-1 intentionally skipped)

src/ZB.MOM.WW.ScadaBridge.Transport/Serialization/BundleSerializer.cs

@@ -1,6 +1,5 @@
 using System.IO.Compression;
 using System.Text.Json;
-using System.Text.Json.Serialization;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Transport;
 using ZB.MOM.WW.ScadaBridge.Transport.Encryption;
 
@@ -26,12 +25,9 @@ public sealed class BundleSerializer
     private const string ContentJsonEntryName = "content.json";
     private const string ContentEncEntryName = "content.enc";
 
-    private static readonly JsonSerializerOptions JsonOptions = new()
-    {
-        WriteIndented = false,
-        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
-        Converters = { new JsonStringEnumConverter() },
-    };
+    // All bundle content serialization goes through BundleJsonOptions.Default —
+    // see that class for the rationale (WhenWritingNull + unknown-member tolerance).
+    private static readonly JsonSerializerOptions JsonOptions = BundleJsonOptions.Default;
 
     /// <summary>
     /// Serializes the bundle content to its canonical JSON byte form. Exposed