feat: FrankenPHP worker-mode compatibility for BackgroundProcess

[HecFranco]

Makes cocur/background-process safe to use under FrankenPHP worker mode (and other long-lived PHP runtimes) by ensuring request-scoped process state does not leak across requests.

Changes

• Implement Symfony\Contracts\Service\ResetInterface and reset() to clear $command / $pid between requests when the instance is a shared service
• Document the recommended DI pattern: register Factory as a shared service and create a fresh BackgroundProcess per operation
• Detect running processes via /proc/<pid> on Linux so isRunning() works in minimal Docker/FrankenPHP images without a ps binary
• Fail fast with a clear RuntimeException when shell_exec is disabled
• Raise the PHP requirement to >= 8.1, add symfony/service-contracts, and modernize the test suite for PHPUnit 10/11

Guidance for consumers

• Do not block a worker thread with while ($process->isRunning()) { sleep(1); } inside an HTTP request
• Persist the PID if the OS process must be managed after the request ends; use BackgroundProcess::createFromPID() later
• Full details: docs/FRANKENPHP.md

Test plan

• docker run --rm -v "$PWD":/app -w /app php:8.3-cli ./vendor/bin/phpunit -c ./
• Review docs/FRANKENPHP.md for shared vs prototype service guidance
• Confirm shell_exec is enabled in target FrankenPHP images (or use Messenger / a queue instead)

[HecFranco]

Hi maintainers,

Thank you for maintaining this library and for keeping it available to the community over the years.

This merge request adds FrankenPHP worker-mode compatibility (ResetInterface / reset(), safer DI guidance, /proc-based process detection on Linux, and a clear guard when shell_exec is disabled) so the package remains reliable in long-lived PHP runtimes.

I would appreciate your review and approval when you have a moment.

Thank you again for your work and support.