IBX-12606: Replaced the Symfony deprecation thresholds with PHPUnit's native deprecation gate - #94

Description:

SYMFONY_DEPRECATIONS_HELPER thresholds are dead under PHPUnit >=10: phpunit-bridge no longer registers a DeprecationErrorHandler, so trigger_deprecation() calls were silently ignored (this repo had it set to disabled=1, which was itself a no-op). This replaces it with PHPUnit 11's native gate: failOnDeprecation + a <source baseline="..."> scoped to src/lib, src/bundle and src/contracts, so CI fails only on NEW deprecations.

The single config (phpunit.xml) is gated, with an empty baseline (0 pre-existing issues, confirmed identical when regenerated under both PHP 8.3 and PHP 8.4), after removing the dead SYMFONY_DEPRECATIONS_HELPER env line. Verified with a trigger_deprecation() probe in src/lib/PurgeClient/LocalPurgeClient.php::__construct() (exercised by tests/lib/PurgeClient/LocalPurgeClientTest.php): the suite correctly failed, then passed again after removal (git diff -- src clean). composer check-cs and phpstan analyse are unchanged (clean).

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

Base branch: 6.0 (this repo's PHPUnit 11 migration PR is already merged).

• ignoreIndirectDeprecations is deliberately false: PHPUnit classifies a Symfony deprecation caused by our code as "indirect" (callee deprecation-contracts/function.php, caller the vendor method), so true would silently drop every deprecated-vendor-API usage. Vendor-originated baseline entries are expected; new ones fail the build until the baseline is refreshed on purpose.
• 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.
• Restored failOnWarning="true" and failOnNotice="true" on phpunit.xml: PHPUnit 9's convertErrorsToExceptions/convertNoticesToExceptions/convertWarningsToExceptions were dropped in PHPUnit 10+ and their replacements default to false, so notices and warnings had silently stopped failing the run.
• Dropped the unused symfony/phpunit-bridge dependency and its SymfonyExtension bootstrap line from phpunit.xml: the extension only wires ClockMock/DnsMock, and its deprecation handler is inert under PHPUnit >=10; the repo has no ClockMock/DnsMock/ExpectDeprecationTrait/expectDeprecation()/SYMFONY_DEPRECATIONS_HELPER usage, so it was dead weight. composer why symfony/phpunit-bridge confirms nothing else in the project requires it.

For QA:

N/A

Documentation:

N/A