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/views/api/v1/conversations/partials/_conversation.json.jbuilder

@@ -7,10 +7,16 @@ json.meta do
     json.partial! 'api/v1/models/contact', formats: [:json], resource: conversation.contact
   end
   json.channel conversation.inbox.try(:channel_type)
-  if conversation.assignee&.account
+  if conversation.assigned_entity.is_a?(AgentBot)
     json.assignee do
-      json.partial! 'api/v1/models/agent', formats: [:json], resource: conversation.assignee
+      json.partial! 'api/v1/models/agent_bot_slim', formats: [:json], resource: conversation.assigned_entity
     end
+    json.assignee_type 'AgentBot'
+  elsif conversation.assigned_entity&.account
+    json.assignee do
+      json.partial! 'api/v1/models/agent', formats: [:json], resource: conversation.assigned_entity
+    end
+    json.assignee_type 'User'
   end
   if conversation.team.present?
     json.team do