Add chat message API endpoints for operators and guests

## Task Add controller endpoints for sending and retrieving chat messages in support sessions. ## Endpoints ### For Operators (authenticated) ```ruby # POST /remote_support/sessions/:id/messages # Body: { content: "message text" } # Returns: { id, content, participant_type, participant_name, created_at } ``` ### For Guests (token-authenticated) ```ruby # POST /support/guest/:token/messages # Body: { content: "message text", guest_name: "optional name" } # Returns: { id, content, participant_type, participant_name, created_at } ``` ### Message History ```ruby # GET /remote_support/sessions/:id/messages # Returns: [{ id, content, participant_type, participant_name, created_at }, ...] # For operators: requires authentication # Accessible even after session ends (for review) ``` ### Guest Message History ```ruby # GET /support/guest/:token/messages # Returns message history for active sessions only ``` ## Controller Structure - Add methods to `RemoteSupportSessionsController` for operator endpoints - Add methods to `RemoteSupportGuestController` for guest endpoints ## Files to Modify - `app/controllers/remote_support_sessions_controller.rb` - `app/controllers/remote_support_guest_controller.rb` - `config/routes.rb` - `spec/requests/remote_support_sessions_chat_spec.rb` - `spec/requests/remote_support_guest_chat_spec.rb`