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/services/conversations/assignment_service_spec.rb

@@ -0,0 +1,60 @@
+require 'rails_helper'
+
+describe Conversations::AssignmentService do
+  let(:account) { create(:account) }
+  let(:agent) { create(:user, account: account) }
+  let(:agent_bot) { create(:agent_bot, account: account) }
+  let(:conversation) { create(:conversation, account: account) }
+
+  describe '#perform' do
+    context 'when assignee_id is blank' do
+      before do
+        conversation.update!(assignee: agent, assignee_agent_bot: agent_bot)
+      end
+
+      it 'clears both human and bot assignees' do
+        described_class.new(conversation: conversation, assignee_id: nil).perform
+
+        conversation.reload
+        expect(conversation.assignee_id).to be_nil
+        expect(conversation.assignee_agent_bot_id).to be_nil
+      end
+    end
+
+    context 'when assigning a user' do
+      before do
+        conversation.update!(assignee_agent_bot: agent_bot, assignee: nil)
+      end
+
+      it 'sets the agent and clears agent bot' do
+        result = described_class.new(conversation: conversation, assignee_id: agent.id).perform
+
+        conversation.reload
+        expect(result).to eq(agent)
+        expect(conversation.assignee_id).to eq(agent.id)
+        expect(conversation.assignee_agent_bot_id).to be_nil
+      end
+    end
+
+    context 'when assigning an agent bot' do
+      let(:service) do
+        described_class.new(
+          conversation: conversation,
+          assignee_id: agent_bot.id,
+          assignee_type: 'AgentBot'
+        )
+      end
+
+      it 'sets the agent bot and clears human assignee' do
+        conversation.update!(assignee: agent, assignee_agent_bot: nil)
+
+        result = service.perform
+
+        conversation.reload
+        expect(result).to eq(agent_bot)
+        expect(conversation.assignee_agent_bot_id).to eq(agent_bot.id)
+        expect(conversation.assignee_id).to be_nil
+      end
+    end
+  end
+end