feat: Add support for realtime-events in copilot-threads and copilot-messages (#11557)

- Add API support for creating a thread
- Add API support for creating a message
- Remove uuid from thread (no longer required, we will use existing
websocket connection to send messages)
- Update message_type to a column (user, assistant, assistant_thinking)

enterprise/app/models/captain/assistant.rb

@@ -29,6 +29,7 @@ class Captain::Assistant < ApplicationRecord
   has_many :inboxes,
            through: :captain_inboxes
   has_many :messages, as: :sender, dependent: :nullify
+  has_many :copilot_threads, dependent: :destroy_async
 
   validates :name, presence: true
   validates :description, presence: true