KRED-2946: This PR adds support for the Midtrans Partner Update Merchant Notification API and improves project documentation.

[philiplambok-mekari]

Summary

This PR adds support for the Midtrans Partner Update Merchant Notification API and improves project documentation.

Changes

New API Endpoint

• Added merchant_update_notification API wrapper for updating merchant notification URLs
• Supports updating: payment, iris, recurring, pay account notifications, and finish payment redirect URL
• Includes full test coverage with WebMock

Documentation Improvements

• CLAUDE.md: Created comprehensive guide for future development with architecture overview and step-by-step instructions for adding new endpoints
• README.md: Restructured with table of contents, improved code examples, added sections for Merchant Management, Disbursements, Utilities, and Error Handling

Technical Details

• API class: lib/midtrans_api/api/merchant/update_notification.rb
• Model class: lib/midtrans_api/model/merchant/update_notification.rb
• Test file: spec/lib/midtrans_api/api/merchant/update_notification_spec.rb
• Added patch HTTP method support to Client

API Reference

https://docs.midtrans.com/reference/partner-update-merchant-notification