Create SendChatMessageService for support session chat

## Task Create service objects for sending messages in support sessions, handling both operator and guest message creation. ## Service: RemoteSupport::SendChatMessageService ```ruby module RemoteSupport class SendChatMessageService < BaseService option :session option :content option :participant_type # :operator or :guest option :user, optional: true # present for operators option :guest_name, optional: true # present for guests def call # Validate session is active/pending (not ended) # Create message record # Trigger broadcast (will be consumed by WebSocket channel) # Return Success(message) or Failure end end end ``` ## Broadcast Mechanism After message creation, trigger a broadcast that: 1. For operators: publishes to NATS `tenants.{tenant}.support_sessions.{id}.messages` 2. For guests: triggers ActionCable broadcast to session channel ## Files to Create - `app/services/remote_support/send_chat_message_service.rb` - `spec/services/remote_support/send_chat_message_service_spec.rb`