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/presenters/conversations/event_data_presenter.rb

@@ -30,7 +30,8 @@ class Conversations::EventDataPresenter < SimpleDelegator
   def push_meta
     {
       sender: contact.push_event_data,
-      assignee: assignee&.push_event_data,
+      assignee: assigned_entity&.push_event_data,
+      assignee_type: assignee_type,
       team: team&.push_event_data,
       hmac_verified: contact_inbox&.hmac_verified
     }