Having trouble setting user session variables when connecting to a MySQL server through ShardingSphere Proxy

```
Welcome to the MariaDB monitor. Commands end with ; or \g.

Your MySQL connection id is 2
Server version: 5.7.22-ShardingSphere-Proxy 5.5.0 Source distribution

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MySQL [(none)]>
MySQL [(none)]>
MySQL [(none)]>
MySQL [(none)]> set @test_var = 1;
ERROR 1193 (HY000): Unknown system variable '@test_var'
MySQL [(none)]> set @test_var := 1;
ERROR 1193 (HY000): Unknown system variable '@test_var'
```

Start by reproducing the reported `set @test_var = 1;` and `set @test_var := 1;` commands through ShardingSphere Proxy against MySQL. Trace how the proxy handles user session variables and establish whether assignment should succeed; done means the behavior is fixed or clearly documented with a regression test or reproducible resolution.

Written by the indexing model from the issue text.