CI: add PHP 8.5 to test matrix

[parisek]

Motivation

Package declares "php": "^8.3"; PHP 8.5 is out (2025-11) and consumers will run it. CI matrix (.github/workflows/tests.yml) currently covers 8.3 + 8.4 only.

Specification

• Add '8.5' to the php matrix in the test job.
• Fix anything it surfaces (deprecations from php-parser/opis/phpunit are the likely candidates); bump dev deps if needed.

Acceptance criteria

• composer check green on 8.3, 8.4 and 8.5 in CI.

[parisek]

Deferred for now — none of our environments run PHP 8.5 yet. Revisit once a consumer project moves to 8.5.