General documentation update

[tsutomi]

This pull request makes extensive updates to the .github/README.md to document new features, connectors, and improvements in the messaging framework, as well as removes the obsolete validation duplication action plan. The most important changes include the addition of Firebase Cloud Messaging support, expanded WhatsApp Business integration, comprehensive webhook support, and improvements to batch processing and performance. The documentation now reflects a more complete and modern feature set for messaging, including new usage examples and connector capabilities.

New Features and Connector Support:

• Added documentation and usage examples for Firebase Cloud Messaging (FCM) connector, including device and topic messaging, multicast support, and rich notifications. [1] [2] [3] [4] F298f254L348R348)
• Expanded WhatsApp Business integration, including webhook support, message receiving, status updates, interactive elements, and media attachments. [1] [2] [3] [4] [5] F298f254L348R348)

Webhook and Batch Processing Enhancements:

• Documented comprehensive webhook support for receiving messages and status updates, including controller examples for Twilio SMS and WhatsApp.
• Added batch processing and performance section, with examples for efficient bulk sending and Firebase multicast. (.github/README.mdL168-R293, F298f254L348R348)

General Documentation Improvements:

• Updated connector capability descriptions, installation instructions, and endpoint type safety examples to reflect new features. [1] [2] [3] [4] [5] [6]
• Expanded testing section to include new connector tests and clarified development requirements. [1] [2]

Code and API Changes:

• Updated connector base class and example implementations to use the new SendResult type instead of MessageResult, aligning with current API.

Cleanup:

• Removed the obsolete VALIDATION_DUPLICATION_FIX_ACTION_PLAN.md file, as validation logic is now centralized and documented in the base connector class.