fix: Handle emoji and special characters in mention notifications (#11857)

Fixes notification display issues when user or team names contain emojis
and special characters. Previously, mention notifications would show
URL-encoded characters instead of properly formatted names with emojis.

  **Before:**
Notification: "John Doe: Hey @%F0%9F%91%8D%20customer%20support please
check this"

  **After:**
  Notification: "John Doe: Hey @👍 customer support please check this"

app/helpers/message_format_helper.rb

@@ -1,9 +1,13 @@
 module MessageFormatHelper
-  include RegexHelper
-
   def transform_user_mention_content(message_content)
     # attachment message without content, message_content is nil
-    message_content.presence ? message_content.gsub(MENTION_REGEX, '\1') : ''
+    return '' unless message_content.presence
+
+    # Use CommonMarker to convert markdown to plain text for notifications
+    # This handles all markdown formatting (links, bold, italic, etc.) not just mentions
+    # Converts: [@👍 customer support](mention://team/1/%F0%9F%91%8D%20customer%20support)
+    # To: @👍 customer support
+    CommonMarker.render_doc(message_content).to_plaintext.strip
   end
 
   def render_message_content(message_content)