chore: Remove message touch:true, use combined update query (#13770)

Removes touch: true from the belongs_to :conversation association on
Message and consolidates the conversation timestamp update into the
existing set_conversation_activity callback.

Previously, every message save triggered two separate UPDATE queries on
the conversation — one from Rails' touch (updating updated_at) and
another from set_conversation_activity (updating last_activity_at). This
combines both into a single update_columns call, reducing write load on
the conversations table on every message creation.

### What changed
- Removed touch: true from belongs_to :conversation in Message
- Added updated_at: created_at to the existing update_columns call in
set_conversation_activity

app/models/message.rb

@@ -125,7 +125,7 @@ class Message < ApplicationRecord
 
   belongs_to :account
   belongs_to :inbox
-  belongs_to :conversation, touch: true
+  belongs_to :conversation
   belongs_to :sender, polymorphic: true, optional: true
 
   has_many :attachments, dependent: :destroy, autosave: true, before_add: :validate_attachments_limit
@@ -429,7 +429,7 @@ class Message < ApplicationRecord
 
   def set_conversation_activity
     # rubocop:disable Rails/SkipsModelValidations
-    conversation.update_columns(last_activity_at: created_at)
+    conversation.update_columns(last_activity_at: created_at, updated_at: Time.current)
     # rubocop:enable Rails/SkipsModelValidations
   end