- Replace all [JsonProperty] attributes with [JsonPropertyName] across all model classes (LabelModel, ExtensionModel, RoleModel, OAuthResponse, OAuthAppAuthorizationResponse, ExtensionScope, RoleModel nested classes, Token models)
- Remove [JsonObject] attributes — null handling covered globally by DefaultIgnoreCondition = WhenWritingNull
- Replace Newtonsoft JsonSerializer/JsonConverter with System.Text.Json equivalents in all service constructors and model classes
- Migrate ParameterCollection.AddQuery(JObject) → AddQuery(JsonNode) and QueryParamValue.JObjectParameterValue to use JsonNode
- Remove OpenJObjectResponse() from IResponse and ContentstackResponse — replaced by OpenJsonObjectResponse() (System.Text.Json.Nodes.JsonObject)
- Rewrite NodeJsonConverterTest and TextNodeJsonConverterTest using STJ public API (JsonSerializer.Serialize/Deserialize)
- Rewrite CustomJsonConverter mock from Newtonsoft JsonConverter base to JsonConverter<object> (STJ)
- Migrate CustomWidgetModel scope serialization from JsonTextWriter to JsonSerializer.Serialize
- Remove Newtonsoft.Json 13.0.3 package reference from core csproj
- Re-enable all previously excluded model files (Role, Taxonomy, Term, Extension, Label, AuditLog, Webhook, Token, CustomExtension) and uncomment corresponding Stack accessor methods
- Re-enable 4 excluded integration test files and fix missing using directives
- Re-enable all 25 excluded unit test files; fix ArgumentException vs InvalidOperationException assertions for uid-empty guard methods
- Enable OAuth: remove Compile Remove for OAuthHandler.cs and Services/OAuth, uncomment OAuth methods and EnsureOAuthTokenIsValidAsync in ContentstackClient
- Fix service constructor calls in Term, Taxonomy, Webhook, Extension, AuditLog — remove legacy serializer first-arg pattern
- Update unit test count: 991 → 1,242 tests all passing
AuditLog,Extension,Label,Role,Taxonomy,Term,Webhook,DeliveryToken,ManagementToken, andCustomExtensionfully re-enabled with STJ attributes; all correspondingStack.*()accessor methods restored[JsonProperty]replaced with[JsonPropertyName]across all model classes;[JsonObject(ItemNullValueHandling)]removed in favour of globalDefaultIgnoreCondition = WhenWritingNullOAuthHandler,OAuthTokenService,OAuthAppAuthorizationService, andOAuthAppRevocationServicenow active; auto token refresh wired intoInvokeAsyncpipeline viaEnsureOAuthTokenIsValidAsyncJsonTextWritertoSystem.Text.Json.JsonSerializer.Serialize()Test002_Should_Create_Dashboard,Test003_Should_Create_Custom_Widget,Test004_Should_Create_Custom_field) now have real implementations usingextension.htmlsourceContentstack012b), DeliveryToken (Contentstack016), Taxonomy (Contentstack017), Role (Contentstack019)