DEV: Refactor a little chat uploads

This is extracted from #22390.

This patch simplifies a little how we handle uploads in chat, relying on
ActiveRecord mechanisms instead of calling custom methods.

This also makes `Chat::Message#validate_message` a “real” AR validation,
meaning it will run automatically when `#valid?` is called.

plugins/chat/app/models/chat/message.rb

@@ -73,9 +73,11 @@ module Chat
 
     before_save { ensure_last_editor_id }
 
+    validate :validate_message
+
     def self.polymorphic_class_mapping = { "ChatMessage" => Chat::Message }
 
-    def validate_message(has_uploads:)
+    def validate_message
       self.message =
         TextCleaner.clean(self.message, strip_whitespaces: true, strip_zero_width_spaces: true)
 
@@ -85,7 +87,7 @@ module Chat
         Chat::DuplicateMessageValidator.new(self).validate
       end
 
-      if !has_uploads && message_too_short?
+      if uploads.empty? && message_too_short?
         self.errors.add(
           :base,
           I18n.t(
@@ -103,23 +105,6 @@ module Chat
       end
     end
 
-    def attach_uploads(uploads)
-      return if uploads.blank? || self.new_record?
-
-      now = Time.now
-      ref_record_attrs =
-        uploads.map do |upload|
-          {
-            upload_id: upload.id,
-            target_id: self.id,
-            target_type: self.class.polymorphic_name,
-            created_at: now,
-            updated_at: now,
-          }
-        end
-      UploadReference.insert_all!(ref_record_attrs)
-    end
-
     def excerpt(max_length: 50)
       # just show the URL if the whole message is a URL, because we cannot excerpt oneboxes
       return message if UrlHelper.relaxed_parse(message).is_a?(URI)

plugins/chat/app/services/chat/channel_view_builder.rb

@@ -184,7 +184,7 @@ module Chat
         context.threads =
           ::Chat::Thread
             .strict_loading
-            .includes(last_message: [:user], original_message_user: :user_status)
+            .includes(last_message: %i[user uploads], original_message_user: :user_status)
             .where(id: messages.map(&:thread_id).compact.uniq)
 
         # Saves us having to load the same message we already have.