Add Symfony Scheduler provider integration

[ohmyfelix]

What changed

• Add experimental Symfony Scheduler provider support via scheduler.providers.
• Discover ScheduleProviderInterface services with #[AsSchedule] and task attributes.
• Register scheduler transports, messenger event listener wiring, and the Symfony debug command.
• Cover provider, task, messenger, console, and invalid-config paths.

Why

This adds an additive integration path for Symfony Scheduler providers while preserving the existing cron job scheduler configuration.

Testing

• composer validate --strict
• vendor/bin/tester -s -p php --colors 1 -C tests/Cases/DI
• make qa
• make tests