Add CONTRIBUTING.md for Azure Generator

eng/packages/http-client-csharp/CONTRIBUTING.md

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+# Contributing to @azure-typespec/http-client-csharp
+
+Welcome! This guide will help you set up your development environment and contribute to the Azure TypeSpec HTTP Client C# package.
+
+## Table of Contents
+
+- [Prerequisites](#prerequisites)
+- [Getting Started](#getting-started)
+- [Development Workflow](#development-workflow)
+- [Testing](#testing)
+- [Code Generation](#code-generation)
+- [Creating Pull Requests](#creating-pull-requests)
+- [Getting Help](#getting-help)
+- [Code of Conduct](#code-of-conduct)
+
+## Prerequisites
+
+Before you begin, ensure you have the following installed:
+
+- [Node.js](https://nodejs.org/) (version 20 or higher)
+- [.NET 9.0 SDK](https://dotnet.microsoft.com/download/dotnet/9.0) (required - see global.json for exact version)
+- [npm](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm)
+- [Git](https://git-scm.com/)
+- [PowerShell](https://docs.microsoft.com/powershell/scripting/install/installing-powershell) (version 7.0 or higher, for advanced testing and code generation scenarios)
+- [Long Path Support](https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=powershell#registry-setting-to-enable-long-paths) (Windows only required to avoid path length limitations during development)
+
+## Getting Started
+
+### 1. Clone the Repository
+
+```bash
+git clone https://github.com/Azure/azure-sdk-for-net.git
+cd azure-sdk-for-net/eng/packages/http-client-csharp
+```
+
+### 2. Install Dependencies
+
+From the repository root:
+
+```bash
+npm ci
+```
+
+### 3. Build the C# Package
+
+```bash
+npm run build
+```
+
+This command runs:
+
+- `npm run build:emitter` - Builds the TypeScript emitter
+- `npm run build:generator` - Builds the .NET generator
+
+### 4. Verify Installation
+
+After building, you can verify everything is working correctly by running:
+
+```bash
+npm run test
+```
+
+## Development Workflow
+
+### Package Structure
+
+The C# HTTP client package consists of two main components:
+
+- **Emitter** (`/emitter`): TypeScript code that processes TypeSpec definitions and produces input for the generator
+- **Generator** (`/generator`): .NET code that generates C# client libraries from the emitter output
+
+### Making Changes
+
+1. **Create a fork** of the repository and clone it:
+
+   ```bash
+   git clone https://github.com/YOUR_USERNAME/azure-sdk-for-net.git
+   cd azure-sdk-for-net
+   git checkout -b feature/your-feature-name
+   ```
+
+2. **Make your changes** to the codebase
+
+3. **Build your changes**:
+
+   ```bash
+   # Build emitter only
+   npm run build:emitter
+
+   # Build generator only
+   npm run build:generator
+
+   # Build everything
+   npm run build
+   ```
+
+4. **Test your changes**:
+
+   ```bash
+   # Test emitter only
+   npm run test:emitter
+
+   # Test generator only
+   npm run test:generator
+
+   # Test everything
+   npm run test
+   ```
+
+### Code Style and Linting
+
+- **Format code**: `npm run prettier:fix`
+- **Run linting**: `npm run lint`
+- **Fix lint issues**: `npm run lint:fix`
+
+## Testing
+
+### Unit Tests
+
+The package includes both TypeScript (emitter) and C# (generator) tests:
+
+```bash
+# Run all tests
+npm run test
+
+# Run emitter tests only (TypeScript/Vitest)
+npm run test:emitter
+
+# Run generator tests only (.NET)
+npm run test:generator
+```
+
+> **Note**: Some tests may require a full workspace build to resolve all dependencies before running successfully.
+
+### Integration Testing with Spector
+
+The package uses the Spector test framework for end-to-end testing of generated code:
+
+```bash
+# Run Spector tests (requires PowerShell)
+./eng/scripts/Test-Spector.ps1
+
+# Run Spector tests with filter (filters by directory path)
+./eng/scripts/Test-Spector.ps1 "http/type/array"
+
+# Get Spector test coverage (outputs to ./generator/artifacts/coverage, not checked in)
+./eng/scripts/Get-Spector-Coverage.ps1
+```
+
+### Test Project Generation
+
+Generate test projects to validate the emitter and generator:
+
+```bash
+# Generate all test projects (requires PowerShell)
+./eng/scripts/Generate.ps1
+
+# Generate specific test project
+./eng/scripts/Generate.ps1 -filter "project-name"
+
+# Generate with stubbed mode disabled
+./eng/scripts/Generate.ps1 -Stubbed $false
+```
+
+## Code Generation
+
+### Regenerating Test Projects
+
+To regenerate test projects after making changes:
+
+**Generate projects**:
+
+```bash
+./eng/scripts/Generate.ps1
+```
+
+### Regenerating Azure Libraries
+
+To regenerate Azure libraries using your local changes, run:
+
+```bash
+./eng/scripts/RegenPreview.ps1
+```
+
+This will regenerate all the Azure libraries and allow you to view any potential diffs your changes may cause. For more information on the script's usage, see [RegenPreview](./eng/scripts/docs/RegenPreview.md).
+
+## Creating Pull Requests
+
+### 1. Prepare Your PR
+
+Before creating a pull request:
+
+- [ ] Ensure all tests pass: `npm run test`
+- [ ] Ensure code is properly formatted: `npm run prettier:fix`
+- [ ] Ensure code is properly linted: `npm run lint:fix`
+- [ ] Generate and test projects: `./eng/scripts/Generate.ps1` (if applicable)
+- [ ] Update documentation if needed
+
+### 2. Create the PR
+
+1. Push your branch to your fork or the main repository
+2. Create a pull request to the [Azure/azure-sdk-for-net](https://github.com/Azure/azure-sdk-for-net) repository
+3. Provide a clear description of your changes
+4. Link any related issues
+
+## Getting Help
+
+- **Issues**: Report bugs or request features in the [Azure SDK for .NET repository issues](https://github.com/Azure/azure-sdk-for-net/issues)
+- **Documentation**: Check the [TypeSpec documentation](https://typespec.io/) for more information
+- **C# Customization**: See the [Customization Guide](https://github.com/microsoft/typespec/blob/main/packages/http-client-csharp/.tspd/docs/customization.md)
+
+## Code of Conduct
+
+This project follows the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/). Please be respectful and inclusive in all interactions.
+
+Thank you for contributing! 🎉