Add C# language support to the Facebook Business SDK Codegen

[BranislavMateas]

Checklist

• [✔ ] I've read the Contributing Guidelines and the Code of Conduct
• [ ✔] I've completed the Contributor License Agreement

Pull Request Details

This PR adds C# language support to the Facebook Business SDK Codegen, enabling automatic generation of the C# SDK for the Facebook Marketing API.

Changes

New C# Language Support

• CodeGenLanguageCs.js: Language processor with C# type mappings and code generation logic
• C# Templates (templates/csharp/):
  • codegen.mustache: Main template for API object classes
  • APIContext.cs.versioned.mustache: API context configuration with template variables
  • APINode.cs.versioned.mustache: Base class for API resources
  • APIRequest.cs.versioned.mustache: HTTP request handling with proper error handling
  • APIConfig.cs.versioned.mustache: Configuration constants
  • FacebookBusinessSDK.csproj.mustache: .NET project file

Configuration Updates

• api_specs/version_path.json: Added C# version detection
• src/common/Utils.js: Added 'csharp' to supported languages
• src/processors/CodeGenLanguages.js: Registered C# processor
• README.md: Updated to reflect 6 language support

Critical Bug Fix

• SpecOverridingProcessor.js: Fixed jsonpath-plus v10.3.0 breaking change
  • This fix unblocks code generation for ALL languages (Java, Python, Node.js, Ruby, PHP)

API Request Improvements

• Added explicit PUT HTTP method handling (was incorrectly defaulting to GET)
• Added JsonException catching for deserialization errors
• Consistent exception handling through APIException wrapper

Source Code Restoration

• Restored src/ directory for language extensibility
• All source files rebuilt to lib/ via npm run build

At the same time, the build is present here - https://github.com/BranislavMateas/facebook-business-sdk-dotnet

Test Plan

Test Plan

1. Codegen Execution

• Run npm install && npm run build successfully
• Execute node lib/CodeGenerator.js csharp generates 952 C# classes without errors
• All files generated in correct structure: src/Meta/Ads/SDK/

2. Generated Code Validation

• APIContext.cs uses dynamic {{api_version}} template variable
• APIConfig.cs contains correct API version, base URLs, and user agent
• APIRequest.cs properly handles GET, POST, PUT, and DELETE methods
• Exception handling catches HttpRequestException and JsonException
• Generated code follows C# naming conventions

3. Configuration Verification

• api_specs/version_path.json has csharp entry pointing to correct version file
• src/common/Utils.js includes 'csharp' in supported languages
• src/processors/CodeGenLanguages.js registers C# processor
• Version detection reads from FacebookBusinessSDK.csproj

4. Bug Fix Verification

• jsonpath-plus v10.3.0 fix works (SpecOverridingProcessor.js uses new API)
• All languages still generate without errors: Java, Python, Node.js, Ruby, PHP

5. Generated SDK Repository

• Generated C# SDK is available at: https://github.com/BranislavMateas/facebook-business-sdk-dotnet
• SDK can be built with dotnet build
• All 952 classes compile without errors
• Package can be created with dotnet pack