[php-src] Issue #21314: Different session garbage collector behavior between PHP 8.3 and PHP 8.5

Issue: https://github.com/php/php-src/issues/21314
Author: sebastien-jj

### Description

**Description**

Hello,

We are observing a difference in the behavior of the file-based session garbage collector between PHP 8.3 and PHP 8.5, using the exact same configuration.

We are not sure whether the current behavior is expected or if this could indicate a regression, so we are seeking clarification.

**Configuration (identical on both versions)**

```
session.gc_probability = 1
session.gc_divisor = 1000
session.gc_maxlifetime = 1440
```

**Observed Behavior**

Under PHP 8.3 :

The number of session files in the storage directory stabilized around ~10,000 files.

Under PHP 8.5 :

The number of session files can grow to 120,000+.

Expired sessions appear to be cleaned much less frequently.

Traffic level and server environment are same.

**Questions**

Has there been any change in the session GC trigger logic between PHP 8.3 and PHP 8.5?

Has the probabilistic calculation (gc_probability / gc_divisor) changed?

Is there any documented change regarding file-based session handling?

### PHP Version

```plain
PHP 8.5.3
```

### Operating System

_No response_