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/models/agent_bot.rb

@@ -21,9 +21,18 @@ class AgentBot < ApplicationRecord
   include AccessTokenable
   include Avatarable
 
+  scope :accessible_to, lambda { |account|
+    account_id = account&.id
+    where(account_id: [nil, account_id])
+  }
+
   has_many :agent_bot_inboxes, dependent: :destroy_async
   has_many :inboxes, through: :agent_bot_inboxes
   has_many :messages, as: :sender, dependent: :nullify
+  has_many :assigned_conversations, class_name: 'Conversation',
+                                    foreign_key: :assignee_agent_bot_id,
+                                    dependent: :nullify,
+                                    inverse_of: :assignee_agent_bot
   belongs_to :account, optional: true
   enum bot_type: { webhook: 0 }