Use the hybrid query cache so parallel GraphQL requests survive Octane

[tomschlick]

Summary

• Default Lighthouse's query cache to hybrid mode so parallel GraphQL requests no longer 500 under Octane.
• The shared cache now stores the query as a PHP source string. A poisoned __PHP_Incomplete_Class entry is treated as a miss, parsed again, and overwritten.
• Keep the query cache off in PHPUnit so the rest of the suite does not write opcache files.

Test plan

• php artisan test --compact tests/Feature/GraphQL
• Confirm LIGHTHOUSE_QUERY_CACHE_MODE is unset in production and bootstrap/cache is writable
• Create several monitors in parallel and confirm the GraphQL creates succeed without a cache clear