Support kernels outside the default App\Kernel namespace

[javihgil]

Problem

CommandRunner and StreamedCommandRunner instantiate App\Kernel directly.

That makes the component depend on the default Symfony application namespace and prevents reuse in applications with a different kernel class location or bootstrap strategy.

Impact

• the component is less reusable than its package name suggests
• applications with a custom kernel namespace cannot use the default runners safely
• testing and extension are harder than necessary

Expected behavior

The kernel boot strategy should be configurable or injected instead of hardcoding App\Kernel.