Add PHP 8.5 compatibility

[lopadova]

What changed

• Keeps the package runtime constraint at php >=5.4 while removing PHP 8.5 deprecations from nullable class/scalar parameters.
• Restores backwards-compatible API aliases covered by the existing tests (Analytics, provider helpers, User::publish, legacy Storage::updateUser argument order).
• Updates the PHPUnit setup to run on modern PHP and adds GitHub Actions coverage for source compatibility and PHP 8.4/8.5 unit tests.

Validation

• composer validate --strict --no-check-lock
• composer test on PHP 8.4
• php85 -d extension_dir=C:\Users\lopad\.config\herd\bin\php85\ext -d extension=php_mbstring.dll vendor\bin\phpunit
• PHP 8.5 source lint for src/**/*.php
• git diff --check

Note: local PHP 8.5 required explicit mbstring loading because the local Herd PHP 8.5 CLI has that extension disabled by default. PHP 5.4 source lint is covered in CI via shivammathur/setup-php, which documents PHP 5.4 support on GitHub-hosted runners.

[lopadova]

CI passed on the source fork branch: https://github.com/padosoft/php-sdk/actions/runs/26479078644

The original repository currently reports no PR checks for this cross-repo branch, so the passing fork run is linked here for review.