[php-src] Issue #21197: Unused php_add_session_var() in ext/session

Issue: https://github.com/php/php-src/issues/21197
Author: arshidkv12

### Description

Hi team,

`php_add_session_var(zend_string *name)` in `ext/session` seems unused internally. 

I’m happy to contribute: we could either mark it deprecated or remove it safely.

Thoughts?

### PHP Version

```plain
PHP 8.6.0-dev (cli) (built: Feb 11 2026 09:31:31) (NTS DEBUG GCOV)
Copyright (c) The PHP Group
Zend Engine v4.6.0-dev, Copyright (c) Zend Technologies
    with Zend OPcache v8.6.0-dev, Copyright (c), by Zend Technologies
```

### Operating System

_No response_