fix(agent-bot): Dispatch webhook event on agent bot assignment (#13975)

When an AgentBot is assigned to a conversation after the first message
has already been received, the bot does not respond because it never
receives any event. The `message_created` event fires before the bot is
assigned, and the bot has no way to know it was assigned.

Chatwoot already dispatches a `CONVERSATION_UPDATED` event when
`assignee_agent_bot_id` changes, but `AgentBotListener` wasn't listening
for it. This fix adds a `conversation_updated` handler so the bot
receives a webhook with the conversation context when assigned.

## How to reproduce

1. Customer sends a message → conversation created, `message_created`
fires
2. System processes the message (adds labels, custom attributes)
3. System assigns an AgentBot to the conversation via API
4. **Before fix:** Bot receives no event and never responds
5. **After fix:** Bot receives `conversation_updated` event with
conversation payload

## What changed

- **`AgentBotListener`**: Added `conversation_updated` handler that
sends the conversation webhook payload to the assigned bot when the
conversation is updated

## How to test

1. Create an AgentBot with an `outgoing_url` pointing to a webhook
inspector (e.g. webhook.site)
2. Send a message to create a conversation
3. Assign the AgentBot to the conversation via API:
   ```
   POST /api/v1/accounts/{id}/conversations/{id}/assignments
   { "assignee_id": <bot_id>, "assignee_type": "AgentBot" }
   ```
4. Verify the bot receives a `conversation_updated` event at its webhook
URL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/listeners/agent_bot_listener.rb

@@ -15,6 +15,14 @@ class AgentBotListener < BaseListener
     agent_bots_for(inbox, conversation).each { |agent_bot| process_webhook_bot_event(agent_bot, payload) }
   end
 
+  def conversation_updated(event)
+    conversation = extract_conversation_and_account(event)[0]
+    inbox = conversation.inbox
+    event_name = __method__.to_s
+    payload = conversation.webhook_data.merge(event: event_name)
+    agent_bots_for(inbox, conversation).each { |agent_bot| process_webhook_bot_event(agent_bot, payload) }
+  end
+
   def message_created(event)
     message = extract_message_and_account(event)[0]
     inbox = message.inbox