MPT-19028 Add helpdesk chat messages service

[jentyk]

Summary

• introduce ChatMessagesService/AsyncChatMessagesService with full CRUD/list mixins and shared config under /public/v1/helpdesk/chats/{chat_id}/messages
• expose chats().messages(chat_id) on both sync and async chat services and wire new resource into tests
• add unit coverage for endpoint wiring/method presence plus dedicated helpdesk chat messages e2e suites covering list/create/update/delete and error paths

Testing

• Not run (not requested)

Closes MPT-19028

• Add ChatMessagesService and AsyncChatMessagesService with full CRUD and collection capabilities
• Mount chat messages service at /public/v1/helpdesk/chats/{chat_id}/messages
• Expose chats().messages(chat_id) factory method on both synchronous and asynchronous chat services
• Add unit test coverage for endpoint wiring and method presence verification
• Add comprehensive end-to-end test suites for list, create, update, delete, and error handling operations