feat: Add support for persistent copilot threads and messages (#11489)

The agents can see the previous conversations with the copilot if needed
with this change. We would have to cleanup the data after a while. For
now, that is not considered.

This PR adds:
- A new model for copilot_threads (intentionally named thread instead of
conversation to avoid confusion), copilot_messages
- Add the controller to fetch previous threads and messages.

config/routes.rb

@@ -58,9 +58,12 @@ Rails.application.routes.draw do
               end
               resources :inboxes, only: [:index, :create, :destroy], param: :inbox_id
             end
-            resources :documents, only: [:index, :show, :create, :destroy]
             resources :assistant_responses
             resources :bulk_actions, only: [:create]
+            resources :copilot_threads, only: [:index] do
+              resources :copilot_messages, only: [:index]
+            end
+            resources :documents, only: [:index, :show, :create, :destroy]
           end
           resources :agent_bots, only: [:index, :create, :show, :update, :destroy] do
             delete :avatar, on: :member