feat: Unity client SDK (continuation of #1105)

[ChiragAgg5k]

Summary

This PR continues the work started by @Fellmonkey in #1105 to add comprehensive Unity SDK support to the SDK generator.

What was done

• Rebased/fetched the original unity-client-sdk branch from Unity client sdk #1105
• Created a new branch unity-client-sdk-continued preserving all original commits from the community PR
• Merged latest master and resolved conflicts (notably in templates/dotnet/Package/Models/Model.cs.twig)

Original PR content (from #1105)

• Unity Language Implementation: Introduces a new Unity language class (src/SDK/Language/Unity.php) that extends the base Language class, providing Unity-specific type mappings, keywords, and code generation logic for C# in Unity environment.
• Unity Template System: Adds a complete set of Unity-specific templates under templates/unity/Assets/ including:
  • Runtime components (Client, Services, Models, Enums, etc.)
  • Editor tools (Setup Assistant, Setup Window)
  • Unity project files (assembly definitions, project settings)
  • Required .NET libraries for Unity compatibility
• Automated Testing Integration: Introduces Unity2021 test support with:
  • Unity2021Test.php test class that integrates with the existing test framework
  • Unity test source files (Tests.cs, Tests.asmdef) for comprehensive SDK testing
  • Docker-based Unity CI testing using unityci/editor:ubuntu-2021.3.45f1-base-3.1.0
• CI/CD Integration: Updates the GitHub Actions workflow to:
  • Include Unity2021 in the test matrix alongside other SDK platforms
  • Set up the UNITY_LICENSE environment variable for Unity Editor automation
  • Enable automated testing of the Unity SDK in the CI pipeline
• Unity Project Structure: Implements proper Unity package structure with:
  • Assets organized under Assets/Runtime/ and Assets/Editor/
  • Required Unity project settings and manifest files

Changes from original PR

• Resolved merge conflicts with current master
• Preserved all original author commits and attribution

Closes #1105

[greptile-apps]

Silent zero-byte upload when stream data is null

In PrepareRequest's multipart path, the "stream" branch wraps everything in if (inputFile.Data is Stream stream). If inputFile.Data is null or not a Stream, the block is silently skipped and fileData stays as the empty array {} declared on line 279. The zero-byte array is then passed to MultipartFormFileSection, uploading an empty file with no error or warning. The "path" case correctly throws FileNotFoundException and the "bytes" case falls back with Array.Empty<byte>(), but neither notifies the caller — the stream case should throw here too.