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>

spec/presenters/conversations/event_data_presenter_spec.rb

@@ -12,8 +12,9 @@ RSpec.describe Conversations::EventDataPresenter do
         additional_attributes: {},
         meta: {
           sender: conversation.contact.push_event_data,
-          assignee: conversation.assignee,
-          team: conversation.team,
+          assignee: conversation.assigned_entity&.push_event_data,
+          assignee_type: conversation.assignee_type,
+          team: conversation.team&.push_event_data,
           hmac_verified: conversation.contact_inbox.hmac_verified
         },
         id: conversation.display_id,