feat: add comprehensive live API testing and contributing documentation

[AlexMikhalev]

Summary

This PR adds comprehensive live API testing infrastructure and detailed contributing documentation to make the project more accessible to contributors.

🚀 What's Added

New CONTRIBUTING.md Documentation

• Development Setup - Step-by-step setup instructions
• Live API Testing Guide - Detailed instructions for running tests with real credentials
• API Key Management - Security best practices for handling credentials
• Code Quality Standards - Formatting and linting guidelines
• Test Structure - Patterns and examples for adding new tests
• Provider Guidelines - Model names and documentation links

Enhanced Live API Test Suite

• Fixed Model Naming - Updated google/gemini-2.0-flash-exp → google/gemini-2.5-flash
• Enabled Core Tests - OpenRouter and Anthropic basic chat, streaming, tool calling
• Comprehensive Coverage - JSON mode, image processing, multiple providers
• Environment Checks - Tests automatically skip if API keys not set

Updated README.md

• Added "Contributing" section with link to CONTRIBUTING.md
• Updated "Links" section to include contributing guide

✅ Validation Results

Live API Tests Status:

• ✅ OpenRouter Basic Chat - Working
• ✅ OpenRouter Streaming - Working
• ✅ OpenRouter Tool Calling - Working
• ✅ OpenRouter JSON Mode - Working
• ✅ OpenRouter Image Processing - Working
• ✅ OpenRouter Multiple Providers - Working
• ✅ Anthropic Basic Chat - Working
• ✅ Anthropic Streaming - Working

Code Quality:

• ✅ All formatting checks pass (cargo fmt)
• ✅ All clippy warnings addressed
• ✅ 8 tests passed, 0 failed

🔧 Usage

Run Live API Tests:

export OPENROUTER_API_KEY="your-key"
export ANTHROPIC_API_KEY="your-key"
cargo test --test live_api_tests -- --nocapture

Code Quality Checks:

cargo fmt --check
cargo clippy --all-targets --all-features -- -W clippy::all

📚 Documentation

The new CONTRIBUTING.md provides everything contributors need:

• Environment setup
• Testing procedures (especially live API tests)
• Security practices for API keys
• Code style guidelines
• Pull request process

This makes the project much more accessible to new contributors and ensures consistent, high-quality contributions.

🎯 Impact

• Lower barrier to entry for new contributors
• Comprehensive testing with real API validation
• Better code quality through documented standards
• Security best practices for API key management
• Sustainable contribution process

The live API testing infrastructure is now production-ready and fully documented!