Docs improvements and ADR cleanup

[lfortunier]

Changed

• docs/phpunit.md : add "Test Class management" section documenting the 1:1 mapping convention between production and test classes; rename fixture examples from .yml to .yaml and use a more meaningful fixture filename (create_simulation.yaml instead of test_check_entity.yml)
• docs/phpstan.md : translate ignoreErrors comment from French to English; clarify the ekino/phpstan-banned-code section title and add an explicit note that only phpstan-disallowed-calls is used
• docs/psalm.md : name psalm-taint-stubs.php explicitly in the custom-stub explanation
• adr/20260709-php-security-audit-migration.md : remove Drupal 7 rows from Table 1 (Drupal API absent in a Symfony stack); drop the "Provided by" column from Table 2 and remove rows already covered by the Symfony plugin description