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>

app/controllers/concerns/ensure_current_account_helper.rb

@@ -25,6 +25,9 @@ module EnsureCurrentAccountHelper
   end
 
   def account_accessible_for_bot?(account)
-    render_unauthorized('Bot is not authorized to access this account') unless @resource.agent_bot_inboxes.find_by(account_id: account.id)
+    return if @resource.account_id == account.id
+    return if @resource.agent_bot_inboxes.find_by(account_id: account.id)
+
+    render_unauthorized('Bot is not authorized to access this account')
   end
 end