feat: Newtonsoft.Json → System.Text.Json + .NET 10 Upgrade on contentstack-model-generator

[OMpawar-21]

Breaking Changes:

• Removed the Newtonsoft.Json dependency entirely. Both the tool's internals and the models/converters it generates now use System.Text.Json.
• Generated model files now use [JsonPropertyName] instead of [JsonProperty], and reference System.Text.Json / System.Text.Json.Nodes / System.Text.Json.Serialization instead of Newtonsoft.Json.
• Generated embedded-object and modular-block converters now inherit System.Text.Json.Serialization.JsonConverter<T> (Read/Write) instead of Newtonsoft's JsonConverter<T> (ReadJson/WriteJson). Regenerating models against an existing project will change these attributes and converters - update consuming code accordingly.
• Requires .NET 10.0 or later (previously .NET 7.0).
• ContentstackException no longer supports legacy BinaryFormatter-based serialization (removed the obsolete SerializationInfo/StreamingContext constructor and GetObjectData override).

Enhancements:

• Upgraded target framework from net7.0 to net10.0.
• Updated CI/CD pipeline to use .NET 10 SDK (actions/setup-dotnet@v4).
• Updated Microsoft.AspNetCore.Mvc.Testing from 9.0.9 to 10.0.0.
• Migrated internal model DTOs (Field, MetaData, Contenttype, StackResponse, ContentStackError, OAuth token response) from [JsonProperty] to [JsonPropertyName].
• Migrated OAuthService token exchange/refresh deserialization to System.Text.Json.
• Migrated the HTTP layer (IResponse, ContentstackResponse, HTTPRequestHandler, ContentstackHttpRequest) from Newtonsoft JObject to System.Text.Json.Nodes.JsonObject.
• Migrated ContentstackClient's serializer settings, response deserialization, and error parsing to System.Text.Json (JsonSerializerOptions, JsonNode).
• Migrated the code-generation templates (using statements, field attributes, link class, embedded-object converter, modular-block converter, and helper class) to emit System.Text.Json-based generated code.
• Fixed reference-field type resolution (GetDatatypeForContentType) to use System.Text.Json.JsonElement instead of the removed Newtonsoft JArray/JObject types.

Bug fix:

• Fixed GetHeader method visibility (private → internal) to allow test access.
• Fixed a template bug where the generated embeddedItems property's nullable annotation was emitted as literal text ({nullableString()}) instead of being applied, for content types with RTE-embedded references.

[github-actions]

We regret to inform you that you are currently not able to merge your changes into the master branch due to restrictions applied by our SRE team. To proceed with merging your changes, we kindly request that you create a pull request from the staging branch. Our team will then review the changes and work with you to ensure a successful merge into the master branch.

[github-actions]

We regret to inform you that you are currently not able to merge your changes into the master branch due to restrictions applied by our SRE team. To proceed with merging your changes, we kindly request that you create a pull request from the staging branch. Our team will then review the changes and work with you to ensure a successful merge into the master branch.