Document thread safety for shared JsonSerializerOptions instances

Co-authored-by: mdellison90-stack <230609064+mdellison90-stack@users.noreply.github.com>

Author: Copilot

src/shared/Core/JsonHelper.cs

@@ -6,10 +6,15 @@ namespace GitCredentialManager
     /// <summary>
     /// Helper class providing common JSON serialization options and utilities.
     /// </summary>
+    /// <remarks>
+    /// The shared JsonSerializerOptions instances should not be modified after initialization
+    /// to ensure thread safety across the application.
+    /// </remarks>
     public static class JsonHelper
     {
         /// <summary>
         /// Gets JSON serializer options configured for case-insensitive property names.
+        /// Do not modify this instance; create a new instance if different options are needed.
         /// </summary>
         public static JsonSerializerOptions CaseInsensitiveOptions { get; } = new JsonSerializerOptions
         {
@@ -19,6 +24,7 @@ public static class JsonHelper
         /// <summary>
         /// Gets JSON serializer options configured for case-insensitive property names
         /// and ignoring null values when writing.
+        /// Do not modify this instance; create a new instance if different options are needed.
         /// </summary>
         public static JsonSerializerOptions CaseInsensitiveIgnoreNullOptions { get; } = new JsonSerializerOptions
         {