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.

enterprise/app/models/copilot_message.rb

@@ -0,0 +1,27 @@
+# == Schema Information
+#
+# Table name: copilot_messages
+#
+#  id                :bigint           not null, primary key
+#  message           :jsonb            not null
+#  message_type      :string           not null
+#  created_at        :datetime         not null
+#  updated_at        :datetime         not null
+#  account_id        :bigint           not null
+#  copilot_thread_id :bigint           not null
+#  user_id           :bigint           not null
+#
+# Indexes
+#
+#  index_copilot_messages_on_account_id         (account_id)
+#  index_copilot_messages_on_copilot_thread_id  (copilot_thread_id)
+#  index_copilot_messages_on_user_id            (user_id)
+#
+class CopilotMessage < ApplicationRecord
+  belongs_to :copilot_thread
+  belongs_to :user
+  belongs_to :account
+
+  validates :message_type, presence: true, inclusion: { in: %w[user assistant assistant_thinking] }
+  validates :message, presence: true
+end