Housekeeping: ignore .idea, stale phpunit cache, upstream PHP 8.4 deprecations

[loevgaard]

Small items collected from the review:

1. .idea/ exists locally but is not in .gitignore; it is untracked only because nobody added it yet. Add /.idea/ or rely on a global gitignore and say so in a contributing note.
2. A stale .phpunit.result.cache sits at the repository root while phpunit.xml.dist sets cacheDirectory=".build/phpunit". It is gitignored; delete it and drop the ignore entry.
3. Running the suite on PHP 8.4 reports three deprecations, all from setono/bot-detection-bundle (BotDetector::__construct() parameter $popular and BotDetectorInterface::isBotRequest() parameter $request are implicitly nullable). These become errors in PHP 9. Fix upstream with explicit ?Type and bump the constraint here. Until then consider a PHPUnit deprecation policy (failOnDeprecation with a baseline, or SYMFONY_DEPRECATIONS_HELPER) so new deprecations from this bundle's own code are not hidden in the noise.
4. codecov/codecov-action runs with a token but nothing consumes the result; either add a Codecov status check or drop the upload (see Test coverage: 46% of lines, 2 of 29 classes fully covered, no end-to-end test #32).