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/services/conversations/assignment_service.rb

@@ -0,0 +1,43 @@
+class Conversations::AssignmentService
+  def initialize(conversation:, assignee_id:, assignee_type: nil)
+    @conversation = conversation
+    @assignee_id = assignee_id
+    @assignee_type = assignee_type
+  end
+
+  def perform
+    agent_bot_assignment? ? assign_agent_bot : assign_agent
+  end
+
+  private
+
+  attr_reader :conversation, :assignee_id, :assignee_type
+
+  def assign_agent
+    conversation.assignee = assignee
+    conversation.assignee_agent_bot = nil
+    conversation.save!
+    assignee
+  end
+
+  def assign_agent_bot
+    return unless agent_bot
+
+    conversation.assignee = nil
+    conversation.assignee_agent_bot = agent_bot
+    conversation.save!
+    agent_bot
+  end
+
+  def assignee
+    @assignee ||= conversation.account.users.find_by(id: assignee_id)
+  end
+
+  def agent_bot
+    @agent_bot ||= AgentBot.accessible_to(conversation.account).find_by(id: assignee_id)
+  end
+
+  def agent_bot_assignment?
+    assignee_type.to_s == 'AgentBot'
+  end
+end