feat: APIs to assign agents_bots as assignee in conversations (#12836)

## Summary
- add an assignee_agent_bot_id column as an initital step to prototype
this before fully switching to polymorphic assignee
- update assignment APIs and conversation list / show endpoints to
reflect assignee as agent bot
- ensure webhook payloads contains agent bot assignee


[Codex
Task](https://chatgpt.com/codex/tasks/task_e_6912833377e48326b6641b9eee32d50f)

---------

Co-authored-by: Pranav <pranav@chatwoot.com>

db/migrate/20251022162159_add_assignee_agent_bot_id_to_conversations.rb

@@ -0,0 +1,5 @@
+class AddAssigneeAgentBotIdToConversations < ActiveRecord::Migration[7.1]
+  def change
+    add_column :conversations, :assignee_agent_bot_id, :bigint
+  end
+end