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,
+        }
       );
     });