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>

spec/requests/api/v1/accounts/base_controller_spec.rb

@@ -0,0 +1,63 @@
+require 'rails_helper'
+
+RSpec.describe 'Api::V1::Accounts::BaseController', type: :request do
+  let(:account) { create(:account) }
+  let(:inbox) { create(:inbox, account: account) }
+  let!(:conversation) { create(:conversation, account: account, inbox: inbox) }
+  let(:agent) { create(:user, account: account, role: :agent) }
+
+  before do
+    create(:inbox_member, inbox: inbox, user: agent)
+  end
+
+  context 'when agent bot belongs to the account' do
+    let(:agent_bot) { create(:agent_bot, account: account) }
+
+    it 'allows assignments via API' do
+      post api_v1_account_conversation_assignments_url(account_id: account.id, conversation_id: conversation.display_id),
+           headers: { api_access_token: agent_bot.access_token.token },
+           params: { assignee_id: agent.id },
+           as: :json
+
+      expect(response).to have_http_status(:success)
+    end
+  end
+
+  context 'when agent bot belongs to another account' do
+    let(:other_account) { create(:account) }
+    let(:external_bot) { create(:agent_bot, account: other_account) }
+
+    it 'rejects assignment' do
+      post api_v1_account_conversation_assignments_url(account_id: account.id, conversation_id: conversation.display_id),
+           headers: { api_access_token: external_bot.access_token.token },
+           params: { assignee_id: agent.id },
+           as: :json
+
+      expect(response).to have_http_status(:unauthorized)
+    end
+  end
+
+  context 'when agent bot is global' do
+    let(:global_bot) { create(:agent_bot, account: nil) }
+
+    it 'rejects requests without inbox mapping' do
+      post api_v1_account_conversation_assignments_url(account_id: account.id, conversation_id: conversation.display_id),
+           headers: { api_access_token: global_bot.access_token.token },
+           params: { assignee_id: agent.id },
+           as: :json
+
+      expect(response).to have_http_status(:unauthorized)
+    end
+
+    it 'allows requests when inbox mapping exists' do
+      create(:agent_bot_inbox, agent_bot: global_bot, inbox: inbox)
+
+      post api_v1_account_conversation_assignments_url(account_id: account.id, conversation_id: conversation.display_id),
+           headers: { api_access_token: global_bot.access_token.token },
+           params: { assignee_id: agent.id },
+           as: :json
+
+      expect(response).to have_http_status(:success)
+    end
+  end
+end