Use constant instead of comment in MySQL2Adapter

Author: rafaelfranca

activerecord/lib/active_record/connection_adapters/mysql2/database_statements.rb

@@ -70,7 +70,7 @@ def perform_query(raw_connection, sql, binds, type_casted_binds, prepare:, notif
                 # but the client doesn't know it.
                 # But we know that this error is safe to retry, so we do so after
                 # getting rid of the originally cached statement.
-                if error.error_number == 1243 # ER_UNKNOWN_STMT_HANDLER
+                if error.error_number == ER_UNKNOWN_STMT_HANDLER
                   if retry_count.positive?
                     retry_count -= 1
                     retry

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -13,6 +13,7 @@ class Mysql2Adapter < AbstractMysqlAdapter
       ER_BAD_DB_ERROR           = 1049
       ER_DBACCESS_DENIED_ERROR  = 1044
       ER_ACCESS_DENIED_ERROR    = 1045
+      ER_UNKNOWN_STMT_HANDLER   = 1243
       ER_CONN_HOST_ERROR        = 2003
       ER_UNKNOWN_HOST_ERROR     = 2005