fix: Ensure message is always present in conversation_created webhook for WhatsApp attachment messages (#12507)

Fixes https://github.com/chatwoot/chatwoot/issues/11753 and
https://github.com/chatwoot/chatwoot/issues/12442

**Problem**
When a WhatsApp conversation started with a media message, the
conversation created webhook would sometimes fire before the message and
its relationships were fully committed to the database. This resulted in
the message being missing
from the webhook payload, breaking external automations that rely on
this field.

**Solution**

Added `ActiveRecord::Base.transaction` wrapper around the core message
processing operations in `Whatsapp::IncomingMessageBaseService` to
ensure atomic execution:

- `set_conversation` (creates conversation)
- `create_messages` (creates message with account_id)
- `clear_message_source_id_from_redis` (cleanup)

Now the webhook only triggers after all related data is fully persisted,
guaranteeing message availability.

app/services/whatsapp/incoming_message_base_service.rb

@@ -32,9 +32,11 @@ class Whatsapp::IncomingMessageBaseService
     set_contact
     return unless @contact
 
-    set_conversation
-    create_messages
-    clear_message_source_id_from_redis
+    ActiveRecord::Base.transaction do
+      set_conversation
+      create_messages
+      clear_message_source_id_from_redis
+    end
   end
 
   def process_statuses