Fix error when searching for URLs that contain the mention syntax (#13151)

Fixes #13150

app/services/account_search_service.rb

@@ -171,7 +171,7 @@ class AccountSearchService < BaseService
   end
 
   def username_complete?
-    query.include?('@') && "@#{query}" =~ Account::MENTION_RE
+    query.include?('@') && "@#{query}" =~ /\A#{Account::MENTION_RE}\Z/
   end
 
   def likely_acct?