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/controllers/api/v1/accounts/conversations/assignments_controller_spec.rb

@@ -15,7 +15,7 @@ RSpec.describe 'Conversation Assignment API', type: :request do
     end
 
     context 'when it is an authenticated bot with out access to the inbox' do
-      let(:agent_bot) { create(:agent_bot, account: account) }
+      let(:agent_bot) { create(:agent_bot) }
       let(:agent) { create(:user, account: account, role: :agent) }
 
       before do
@@ -36,6 +36,7 @@ RSpec.describe 'Conversation Assignment API', type: :request do
 
     context 'when it is an authenticated user with access to the inbox' do
       let(:agent) { create(:user, account: account, role: :agent) }
+      let(:agent_bot) { create(:agent_bot, account: account) }
       let(:team) { create(:team, account: account) }
 
       before do
@@ -54,6 +55,25 @@ RSpec.describe 'Conversation Assignment API', type: :request do
         expect(conversation.reload.assignee).to eq(agent)
       end
 
+      it 'assigns an agent bot to the conversation' do
+        params = { assignee_id: agent_bot.id, assignee_type: 'AgentBot' }
+
+        expect(Conversations::AssignmentService).to receive(:new)
+          .with(hash_including(conversation: conversation, assignee_id: agent_bot.id, assignee_type: 'AgentBot'))
+          .and_call_original
+
+        post api_v1_account_conversation_assignments_url(account_id: account.id, conversation_id: conversation.display_id),
+             params: params,
+             headers: agent.create_new_auth_token,
+             as: :json
+
+        expect(response).to have_http_status(:success)
+        expect(response.parsed_body['name']).to eq(agent_bot.name)
+        conversation.reload
+        expect(conversation.assignee_agent_bot).to eq(agent_bot)
+        expect(conversation.assignee).to be_nil
+      end
+
       it 'assigns a team to the conversation' do
         team_member = create(:user, account: account, role: :agent, auto_offline: false)
         create(:inbox_member, inbox: conversation.inbox, user: team_member)
@@ -125,7 +145,7 @@ RSpec.describe 'Conversation Assignment API', type: :request do
       end
 
       it 'unassigns the assignee from the conversation' do
-        params = { assignee_id: 0 }
+        params = { assignee_id: nil }
         post api_v1_account_conversation_assignments_url(account_id: account.id, conversation_id: conversation.display_id),
              params: params,
              headers: agent.create_new_auth_token,