Incorrect @phpstan-return annotation on processSLO method

[mylene-lalechere]

The @phpstan-return annotation on the processSLO method in src/Saml2/Auth.php (line 273) is incorrect.

Current annotation:

  @return string|null                                                                                                                                                                                                    
  @phpstan-return ($stay is true ? string : never)          

Problem:

This annotation states that when $stay is false, the method never returns (never). However, the method can return null implicitly in several code paths:

1. When $_GET['SAMLResponse'] is set (lines 281-300) - no explicit return, so returns null
2. When $_GET['SAMLRequest'] is set but invalid (lines 304-307) - no explicit return, so returns null

Only when $_GET['SAMLRequest'] is valid does the method call redirectTo() (line 336), which follows the documented behavior.

Expected behavior:

The @phpstan-return should account for the implicit null returns, or be removed entirely since @return string|null is more accurate.