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/javascript/dashboard/api/inbox/conversation.js

@@ -63,10 +63,9 @@ class ConversationApi extends ApiClient {
   }
 
   assignAgent({ conversationId, agentId }) {
-    return axios.post(
-      `${this.url}/${conversationId}/assignments?assignee_id=${agentId}`,
-      {}
-    );
+    return axios.post(`${this.url}/${conversationId}/assignments`, {
+      assignee_id: agentId,
+    });
   }
 
   assignTeam({ conversationId, teamId }) {

app/javascript/dashboard/api/specs/inbox/conversation.spec.js

@@ -92,8 +92,10 @@ describe('#ConversationAPI', () => {
     it('#assignAgent', () => {
       conversationAPI.assignAgent({ conversationId: 12, agentId: 34 });
       expect(axiosMock.post).toHaveBeenCalledWith(
-        `/api/v1/conversations/12/assignments?assignee_id=34`,
-        {}
+        `/api/v1/conversations/12/assignments`,
+        {
+          assignee_id: 34,
+        }
       );
     });
 

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,

app/javascript/dashboard/routes/dashboard/conversation/ConversationAction.vue

@@ -84,7 +84,7 @@ export default {
         return this.currentChat.meta.assignee;
       },
       set(agent) {
-        const agentId = agent ? agent.id : 0;
+        const agentId = agent ? agent.id : null;
         this.$store.dispatch('setCurrentChatAssignee', agent);
         this.$store
           .dispatch('assignAgent', {