Major refactor: Convert to functional API and improve architecture

[oryono]

Summary

This PR implements a major refactoring of the MTN MoMo Elixir library to modernize its architecture and follow Elixir best practices.

Fixes #11

Changes Made

🔥 Breaking Changes

• Remove GenServer-based API → Replace with functional approach
• Remove Collection/Disbursement modules → Replace with Collections/Disbursements
• Change API from stateful to stateless → Pass config to each function call

✨ New Features

• Environment variable support via new MomoapiElixir.Config module
• Production configuration with proper base URLs
• Comprehensive validation with structured error responses
• Environment-specific config (dev/test/prod)

🚀 Improvements

• Modern configuration - Replace deprecated Mix.Config with Config
• Better error handling - Use {:ok, result} / {:error, reason} patterns throughout
• Comprehensive documentation - Add typespecs and examples
• Remove code duplication - DRY principles between collections and disbursements
• Better testability - Configurable HTTP client with proper mocking

API Changes

Before (GenServer)

# Complex setup required
options = %Collection.Option{subscription_key: "key", user_id: "user", api_key: "api"}
Collection.start(options)
Collection.request_to_pay(body)  # Global state!

After (Functional)

# Simple function calls
config = %{subscription_key: "key", user_id: "user", api_key: "api"}
{:ok, reference_id} = MomoapiElixir.Collections.request_to_pay(config, body)

Environment Variables (New!)

export MOMO_SUBSCRIPTION_KEY="your_key"
export MOMO_USER_ID="your_user_id" 
export MOMO_API_KEY="your_api_key"

{:ok, config} = MomoapiElixir.Config.from_env()
{:ok, reference_id} = MomoapiElixir.Collections.request_to_pay(config, payment)

Technical Improvements

• ✅ Proper validation - Returns all errors at once with detailed context
• ✅ Production ready - Separate prod config with https://momoapi.mtn.com
• ✅ Better testing - Behavior-based HTTP client mocking
• ✅ Type safety - Comprehensive typespecs throughout
• ✅ Documentation - Examples and usage guides
• ✅ Configuration - Environment-specific settings

Testing

• ✅ Updated all existing tests to work with new functional API
• ✅ Added comprehensive validator tests
• ✅ Tests validate environment variable configuration
• ✅ Proper mocking with behavior-based HTTP client

Configuration Files Added/Updated

• 📄 config/prod.exs - NEW: Production configuration
• 📝 config/config.exs - Updated to use modern Config
• 📝 config/dev.exs - Added base_url configuration
• 📝 config/test.exs - Updated with proper test settings

Migration Guide

For existing users, update your code:

# Old way
options = %Collection.Option{subscription_key: "key", user_id: "user", api_key: "api"}
Collection.start(options)
result = Collection.request_to_pay(body)

# New way
config = %{subscription_key: "key", user_id: "user", api_key: "api", target_environment: "sandbox"}
{:ok, reference_id} = MomoapiElixir.Collections.request_to_pay(config, body)

Impact

This refactor makes the library:

• Easier to use - No GenServer setup required
• More testable - Pure functions, easy to mock
• Production ready - Proper configuration and error handling
• Modern Elixir - Follows current best practices
• Better documented - Comprehensive docs and examples

Test Plan

• Manual testing with sandbox API
• Performance benchmarking
• Integration testing in sample application
• Documentation review

Ready for review! 🚀