feat: migrate tests to Testo, require PHP 8.2

[roxblnfk]

What was changed

• Minimum PHP is now 8.2; vimeo/psalm is ^6 || ^7 so it installs on the whole supported range.
• Tests run on Testo instead of PHPUnit; phpunit/phpunit and phpunit.xml are gone, composer test runs testo.
• Mocks stay on Mockery via testo/bridge-mockery. The plugin verifies Mockery expectations after each test, which the PHPUnit suite never did (no Mockery::close()), so the once()/withArgs() expectations are now actually checked.
• CI calls spiral/gh-actions testo.yml (feat(testo): test PHP 8.2-8.6 by default, add test-server input spiral/gh-actions#10): PHP 8.2–8.6, highest dependencies.

Why?

Testo requires PHP 8.2; moving the suite to it is the first step before coverage, mutation testing and the static analysis pass.

Checklist

• Tested
  • Testo suite run locally: 9 tests before (PHPUnit) and after (Testo), the same methods

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