IBX-12606: Replaced the Symfony deprecation thresholds with PHPUnit's native deprecation gate

[ViniTou]

🎫 Issue	IBX-12606

Description:

SYMFONY_DEPRECATIONS_HELPER thresholds are dead under PHPUnit ≥10 (phpunit-bridge's bootstrap returns early and registers no deprecation handler), so @-silenced trigger_deprecation() calls went unchecked. This repo had already migrated to PHPUnit 11 with no SYMFONY_DEPRECATIONS_HELPER env line left to remove, so this PR only adds the native gate: failOnDeprecation="true" + displayDetailsOnTestsThatTriggerDeprecations="true" on phpunit.xml.dist, plus a <source ignoreIndirectDeprecations="false" ignoreSuppressionOfDeprecations="true" baseline="phpunit.baseline.xml"> including rules/ (this repo's real production source dir per composer.json psr-4, not src/).

Only one config exists (phpunit.xml.dist, suite rules) and it gates cleanly: baseline is empty (0 issues, 0 internal / 0 external) — the 4-test suite currently triggers no deprecations. No suites were left ungated.

Caveat found while probing: PHPStan's own FileAnalyser::collectErrors() installs a private error handler around each file analysis (vendor/phpstan/phpstan/phpstan.phar, Analyser/FileAnalyser.php), and it does not forward suppressed/deprecated errors to any previously-registered handler. Since this repo's rule classes only run inside that analysis loop (via processNode(), invoked by PHPStan\Testing\RuleTestCase), a deprecation triggered from inside a rule's processNode() body is silently swallowed and will not fail the build — this is PHPStan's own testing-harness behavior, not something we patched or worked around (vendor code, out of scope). A deprecation triggered anywhere else that PHPUnit exercises directly (rule constructors, test/fixture code, library calls outside the analysis loop) is caught correctly, confirmed via probe (see PR conversation / task report for detail).

To refresh the baseline: ensure vendor/ is a real (non-symlinked) directory with PHPUnit 11 installed, then run vendor/bin/phpunit -c phpunit.xml.dist --generate-baseline phpunit.baseline.xml.

Merge note: this repo's PHPUnit 11 migration is already merged to 6.0, so this branch is built directly off origin/6.0 and is not stacked on any other open PR.

• Baselines are generated on PHP 8.4 (CI's highest matrix version); PHP-native deprecations from vendor code (e.g. league/flysystem implicit-nullable parameters) only fire there. Regenerate with the highest PHP version in the matrix.

Additionally, in this pass:

• Restored failOnNotice="true" on phpunit.xml.dist (alongside the already-present failOnWarning="true"), matching the PHPUnit 9 convertNoticesToExceptions/convertWarningsToExceptions strictness that PHPUnit 10 removed as flags — without it, notices and warnings would no longer fail the build.
• No symfony/phpunit-bridge dependency exists in this repo (composer.json, no Symfony\Bridge\PhpUnit usage, no ClockMock/DnsMock/ExpectDeprecationTrait), so there is nothing to drop and no expectDeprecation() calls to convert.

For QA:

N/A

Documentation:

N/A