Address PHPUnit 12 deprecations

Problem/Motivation

PHPUnit 10 deprecates test metadata written in doc-comments; support is removed entirely in PHPUnit 12. Every test in the module still declares @group, @covers, @coversDefaultClass and @dataProvider as doc-comment annotations, so the test run emits a deprecation for each one, e.g.:

Metadata found in doc-comment for method Drupal\Tests\graphql_webform\Kernel\DataProducer\WebformSubmissionPropertyTest::testResolveNullId(). Metadata in doc-comments is deprecated and will no longer be supported in PHPUnit 12. Update your test code to use attributes instead.

These are surfaced as deprecations only, so the suite still passes. The GitLab CI phpunit job does not fail on deprecations, notices, warnings or risky tests, so this kind of regression slips through unnoticed.

Steps to reproduce

Run the test suite and observe the deprecation notices in the output:

ddev phpunit web/modules/custom/graphql_webform/tests

Proposed resolution

Convert all doc-comment test metadata to PHP attributes from the PHPUnit\Framework\Attributes namespace, across every file under tests/ (41 files affected):

• @group graphql_webform → #[Group('graphql_webform')]
• @coversDefaultClass \Some\Class plus @covers ::method → #[CoversClass(Class::class)], or #[CoversMethod(Class::class, 'method')] where a single method is covered
• @dataProvider providerName → #[DataProvider('providerName')]

Then amend the CI phpunit job so the pipeline fails on all PHPUnit issues, preventing the deprecations from regressing. The DrupalCI phpunit job already fails on test failures; pass PHPUnit's fail-on flags through _PHPUNIT_EXTRA in .gitlab-ci.yml:

phpunit:
  variables:
    _PHPUNIT_EXTRA: '--fail-on-deprecation --fail-on-phpunit-deprecation --fail-on-notice --fail-on-warning --fail-on-risky'

Note: DrupalCI sets SYMFONY_DEPRECATIONS_HELPER to disabled, so Symfony-level deprecations are intentionally out of scope here — re-enabling them would surface unrelated deprecations from Drupal core and contrib. This change targets PHPUnit's own deprecations, notices, warnings and risky tests.

Remaining tasks

• Convert doc-comment metadata to attributes in all test files.
• Amend .gitlab-ci.yml to fail on the issues above.
• Run phpunit, phpcs and phpstan; confirm a clean run with no deprecations.
• Review and commit.