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/javascript/dashboard/components/widgets/conversation/ReplyBoxBanner.vue

@@ -30,7 +30,7 @@ const assignedAgent = computed({
     return currentChat.value?.meta?.assignee;
   },
   set(agent) {
-    const agentId = agent ? agent.id : 0;
+    const agentId = agent ? agent.id : null;
     store.dispatch('setCurrentChatAssignee', agent);
     store.dispatch('assignAgent', {
       conversationId: currentChat.value?.id,