feat(facebook): use HUMAN_AGENT tag for Messenger replies when human-agent config is enabled (#13690)

This PR updates Facebook Messenger outbound tagging in Chatwoot to
support Human Agent messaging when enabled.

Previously, Facebook outbound text and attachment messages were always
sent with:

```
messaging_type: MESSAGE_TAG
tag: ACCOUNT_UPDATE
```
With this change, the tag is selected dynamically:

```
HUMAN_AGENT when ENABLE_MESSENGER_CHANNEL_HUMAN_AGENT is enabled
ACCOUNT_UPDATE as fallback when the flag is disabled
```

app/services/facebook/send_on_facebook_service.rb

@@ -49,7 +49,7 @@ class Facebook::SendOnFacebookService < Base::SendOnChannelService
       recipient: { id: contact.get_source_id(inbox.id) },
       message: fb_text_message_payload,
       messaging_type: 'MESSAGE_TAG',
-      tag: 'ACCOUNT_UPDATE'
+      tag: message_tag
     }
   end
 
@@ -90,10 +90,14 @@ class Facebook::SendOnFacebookService < Base::SendOnChannelService
         }
       },
       messaging_type: 'MESSAGE_TAG',
-      tag: 'ACCOUNT_UPDATE'
+      tag: message_tag
     }
   end
 
+  def message_tag
+    @message_tag ||= GlobalConfigService.load('ENABLE_MESSENGER_CHANNEL_HUMAN_AGENT', nil) ? 'HUMAN_AGENT' : 'ACCOUNT_UPDATE'
+  end
+
   def attachment_type(attachment)
     return attachment.file_type if %w[image audio video file].include? attachment.file_type