feat: migrate .NET templates to System.Text.Json + add .NET 9.0 tests

[Fellmonkey]

What does this PR do?

This pull request introduces two major improvements to the .NET SDK generator:

1. Refactors from Newtonsoft.Json to System.Text.Json for serialization/deserialization - A comprehensive migration that modernizes the .NET SDK by replacing the legacy Newtonsoft.Json library with Microsoft's System.Text.Json library. This includes:

   • Updated package dependencies in Package.csproj.twig
   • Migrated all serialization/deserialization logic in Client.cs.twig, Query.cs.twig, and Extensions.cs.twig
   • Updated model attributes from [JsonProperty] to [JsonPropertyName] in Model.cs.twig
   • Created new ObjectToInferredTypesConverter.cs.twig for proper object type handling
   • Replaced JsonConverter with JsonConverter<object> in ValueClassConverter.cs.twig
   • Updated error handling to use JsonDocument instead of JObject

2. Adds initial test support for .NET 9.0 SDK - Extends platform compatibility by adding test infrastructure for the latest .NET version:

   • Created DotNet90Test.php test class for .NET 9.0 SDK validation
   • Added Tests90.csproj project file targeting .NET 9.0 framework

Test Plan

Regenerate .NET SDKs using the updated templates and verify that existing client code continues to work without changes

Related PRs and Issues

(appwrite/sdk-for-dotnet#48)

Have you read the Contributing Guidelines on issues?

Yes

[Fellmonkey]

Also
update user-agent string interpolation to use string interpolation syntax
(appwrite/sdk-for-dotnet#63)
Generated code after changes:

[Copilot]

Pull Request Overview

This pull request migrates the .NET SDK from Newtonsoft.Json to System.Text.Json while also adding support for .NET 9.0 tests. Key changes include the conversion of JSON serialization/deserialization logic and model attributes, updates to project dependencies and target frameworks, and the addition of new test files for .NET 9.0.

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/languages/dotnet/Tests90.csproj	New project file targeting .NET 9.0
tests/DotNet90Test.php	New test class to validate the .NET 9.0 SDK functionality
templates/dotnet/README.md.twig	Updated documentation to reference System.Text.Json and the new attribute naming
templates/dotnet/Package/Query.cs.twig	Updated Query implementation to use System.Text.Json for serialization/deserialization
templates/dotnet/Package/Package.csproj.twig	Updated target frameworks and package references (switching from Newtonsoft.Json to System.Text.Json)
templates/dotnet/Package/Models/Model.cs.twig	Replaced JsonProperty with JsonPropertyName and adapted deserialization logic
templates/dotnet/Package/Models/InputFile.cs.twig	Set default initializers for properties; note the change for Data initialization
templates/dotnet/Package/Extensions/Extensions.cs.twig	Updated extension methods to use System.Text.Json serialization
templates/dotnet/Package/Converters/ValueClassConverter.cs.twig	Migrated ValueClassConverter to use the new JSON APIs
templates/dotnet/Package/Converters/ObjectToInferredTypesConverter.cs.twig	Added a new converter for inferring object types with System.Text.Json
templates/dotnet/Package/Client.cs.twig	Updated client logic including error handling, file upload validations, and JSON serialization

Comments suppressed due to low confidence (1)

templates/dotnet/Package/Models/InputFile.cs.twig:12

• Initializing Data with a new object() may lead to runtime casting issues when InputFile.Data is expected to be a Stream or byte array. Consider making Data nullable or initializing it to null instead.

public object Data { get; set; } = new object();

[ChiragAgg5k]

@Fellmonkey Thank you for submitting this PR! This feature has been highly requested by our community, so we really appreciate you taking the initiative to implement it.

I noticed that the .NET SDK build is currently failing in the test suite. Could you please take a look at the failing tests and address any issues? Once those are resolved, we'll be happy to move forward with the review process.

Thanks again for your contribution!

[Fellmonkey]

Add missing ObjectToInferredTypesConverter.cs template configuration

Previously forgot to include the ObjectToInferredTypesConverter.cs template in the files array, which caused tests to fail due to missing converter dependency.

[ChiragAgg5k]

@Fellmonkey Great work! 🔥 All tests are passing. We'll just need a review from one of our more experienced .NET engineers before we can merge this.