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)

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

app/views/api/v1/accounts/conversations/assignments/create.json.jbuilder

@@ -1,4 +0,0 @@
-json.payload do
-  json.assignee @conversation.assignee
-  json.conversation_id @conversation.display_id
-end